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What To Do When An RV Waste Tank (Black/Grey) Won’t Drain

Reasons Black & Grey RV Holding Tanks Clog

So you’ve taken your RV to the dump only to find that the black or grey tank will only empty a few gallons or not at all. This is a common problem for RV owners especially those who are dumping a used RV or trailer for the first time. Improper RV holding tank maintenance is one of the main causes for clogged black tanks and the previous owner may not have taken proper care of their RV waste tanks.

There are three main causes of blocked or clogged RV waste tanks.

  1. Broken Gate Valve or T-Handle
  2. Inorganic Blockage (flushable wipes, foreign objects, things like that)
  3. Organic Blockage (valve was left open for a long time causing things to dry and harden creating a blockage)

In this article, I’ll go through some of the main things you can do to unclog a black or grey tank yourself. I’ll start with things you can do with limited equipment in case you are standing at the RV dump station right now reading this article trying to find ways to unclog your black tank.

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How To Unclog An RV Black Tank

No matter what the reason is for a clogged black tank there are a few things you can try right now to get things flowing again. If none of these things work you may need to resort to the more time consuming methods outlined further on in this article.

Make Sure The Gate Valve Is Open

The first thing you need to check for is if the gate valve is open or not. Unless you have a serious hard buildup and blockage right at the valve opening of your black tank there should be at least some liquid that comes out. If absolutely nothing comes out when you open your black tank they problem may be the gate valve.

While it’s not a common problem it’s worth checking out. You should be able to see the gate pull out when you pull on the t-handle or if you have a trailer like mine, where you can’t see the gate move in or out the t-handle should have a bit of resistance when you pull open the gate valve.

One reason for a grey of black tank that won't drain is a broken gate valve.

If the t-handle can move in or out very easily and seems like it’s not attached to anything that means the gate valve isn’t opening. If the t-handle is broken you may have to take some pliers and open the gate valve yourself. If the waste tank starts to drain like normal you have found your issue.

Backflush It

The most common problem will be a clog of something right at the valve opening. If you are lucky it’s just a buildup of toilet paper and other organic stuff (aka number two). You need to flush the tank out backward from the valve opening to displace the clog and hopefully break it apart.

If you have a black tank flush on your RV, travel trailer, or 5th-wheel this may not work because you need to attack the clog from the other end. I suggest always having a sewer tank rinser like the Camco Rhino Blaster Pro W/Gate Valve (click to view on Amazon) for an emergency situation like this one.

Camco Rhino Blaster Sewer Tank Rinser with Gate Valve - Securely Attaches to Your RV Sewer Outlet to Jet Rinse Out Residue, Black Flow Preventer Prevents Hose Contamination - (39085)

Holding tank backflush products like these can be useful in holding tank maintenance as well as for breaking up or even snaking out clogs.

If you already have one of these attach it and start shooting water into the clogged black tank, it should either break up the clog or at least move it to allow the rest of the black tank to drain.

See Also: Best RV Portable Waste Tanks For Black & Grey Water

If you don’t have a black tank flush attachment like this you can also try to flush water into the black tank by opening the grey tank all the way for a few seconds. When both valves are open some of the water from the grey tank should flow into the black tank and put pressure on the clog. If you are lucky it will break it up.

Try opening the grey tank valve while the black tank valve is open a few times. If the grey tank is empty, fill it up all the way so you can get a lot of pressure when you open it. Hopefully, this method will get your black tank draining again.

Check The Roof Vent

If backflushing didn’t work there are still a few things you can check or try before getting into the more time consuming methods for unblocking a clogged black tank.

If your black tank is slowly draining but not flowing out as fast as it should you may have a clogged air vent. This issue is more common than you think especially if you are camping or store your RV in an area with lots of wasps, they may have built a nest in your vent.

You can check this by having someone go into the RV or trailer and flush the toilet while the water pump is off. If you black tank suddenly starts to flow more and it seems like a vacuum seal was broken by allowing air into the tank then you know the air vent is clogged.

Once you’ve emptied your black tank with the toilet open you can try and unclog the air vent by removing the cap to it on the roof and sticking a hose down to try and break up whatever is clogging it. If your roof cap was on properly it shouldn’t be anything more than a wasps nest or some toilet paper than found its way into the vent.

Dump Water Into The Toilet

If backflushing the tank sort of worked and there is at least some blackwater draining out of the black tank another thing you can try is to dump a 5 gallon bucket of water into the toilet. The high velocity of water flushing into the black tank all at one time should break up the clog even more or possibly force out any foreign objects that were blocking it in the first place.

Don’t start with this method as it could make the clog worse. Only do this if you are getting some blackwater to come out of your black tank after backflushing it.

The Drive Around Method

If you’ve tried all of these things and the clog still won’t loosen up you are going to have to use water, chemicals, and motion to try and loosen up the blockage. If you have recently bought a used trailer and haven’t taken it camping yet you should do this to get rid of anything in the RV waste tanks the previous owner may have left you with.

You also should do this to get your black tank good and clean before winterizing it as well.

First, make sure the black tank valve is closed, some people don’t realize they are open and have been using their black tank which is a big reason clogs happen in the first place.

See Also: Best RV/Camper Sewer Hose + Storage, Support, Fittings

Next, you are going to need some chemicals. You should always be using some sort of black tank treatment like or better yet Happy Campers Organic RV Holding Tank Treatment (click to view on Amazon).

You can find the Camco RV toilet treatment in almost any hardware, or superstore but if you have the chance to get the Happy Camper RV Tank Treatment I recommend that one. It may say organic on the packaging but it’s one of the best RV toilet treatments out there and it can break down clogs like nothing else.

Get your 5 gallon bucket and fill it with hot water and about a half a cup of dishwashing soap. Cold water will work as well but hot water is going to be more effective. Take the RV holding tank treatment and dump about 2-3 times more than you normally would down the toilet.

Next, dump the 5 gallons of hot water+dishsoap down the toilet as well. If you have enough hot water to dump a few more 5 gallon buckets down the toilet it’s a good idea to do that, especially if your waste tank is low on water. If not dump at least one more bucket of water down the toilet even if it’s cold water.

It’s time to take your RV or trailer for a ride. You want to get the water sloshing around really good to break up anything and everything in your holding tank. You don’t have to go crazy and drive 50 mph down a washboard dirt road but getting the trailer moving is going to be your best bet at breaking down the clog.

If you have the time you can even let your trailer sit for a few days to let the chemical do the work. If you don’t have the time for that going for a good long drive may be all it takes.

Once you’ve driven around try and dump your black tank again. Hopefully, this has broken down any organic blockage or caused whatever was blocking the valve to come out. You may need to implement some of the steps above to get things going.

Snake It

There is one more thing you can do before calling a plumber and possibly taking apart the entire black tank valve system, you can snake it. Just as a warning if you have never used a drain snake before you need to be really careful to not cause the snake to get bunched up in the pipe because this could make the snake get stuck as well. Everything should work fairly easily and never use excessive force.

The obvious issue with snaking your black tank from the gate valve end is if you break up the clog you are going to have a ton of nasty raw sewage pouring out very quickly and you will have a giant mess on your hands. There are a few things you can do to minimize the mess and contain the blackwater coming out.

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The simplest thing people do is drill a hole on the top of their sewer hose near the bayonet coupling. This will, of course, destroy your sewer hose and you are going to need a backup one handy. You can get a cheap sewer hose for this so you don’t destroy your nice one.

Drill the hole for the drain snake at the top of the sewer hose closest to the connector.

Once you’ve drilled a hole in the top you can slip the snake in and start working it into the black tank. Most black tanks don’t have very much distance between the main opening and the gate valve so a short snake like the Ridgid 3-Foot Toilet Auger (click to view on Amazon) with a 1/2 inch snake should work.

Once you’ve got the main part of the snake through the hole you’ve made I suggest using a rubber glove or at least a rag to cover as much of the hole you are putting the cable through to stop sewage from coming out. Now you can start snaking.

Once the clog has been hit and broken up sewage should start flowing out right away. Even though it’s going to be a mess let some it drain out to make sure the clog is released. You don’t need to dump your entire black tank with the compromised sewer hose. Once the clog has drained out close the gate valve and switch the bad sewer hose with a good one and finish dumping your black tank.

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Another thing I’ve seen people do is use their RV black tank flush attachment to feed the drain snake through. You may need to drill the part the hose attaches to make it big enough to allow the snake through. This method works as well and you might not have to damage a sewer hose if you do this. The drawback to this method is you may damage the RV black tank flush attachment if you are not careful and they tend to be more expensive than a new sewer hose.

Call The Professionals

If none of these things work you may have a big problem on your hands and you will either have to take your RV or trailer to an RV repair shop to get it fixed or call a plumber. Neither of those options sound great but if you’ve tried all of these things and nothing has worked you may be out of options.

How To Unclog An RV Grey Waste Tank

It’s less common for RV grey tanks to clog but it does happen. You can use any of these methods used to unclog an RV black tank. The only thing I would change would be if you are going to use the water in the black tank to try and flush out a clog in the grey tank make sure the black tank has been fully drained and cleaned as good as you can. You don’t want raw sewage going into your grey tank.

You can do this by emptying your black tank and flushing it out with a Camco Swivel Stick Black Tank Sprayer (click to view on Amazon) and also dump a 5 gallon bucket full of water and dish soap down the toilet a few times to clean it out good. Then fill the black tank with fresh water so you have something to flush into the clogged grey tank.

There is also no direct opening to the grey tank like there is to the black tank. It’s going to be harder to get a ton of water into the tank all at once. First put holding tank treatment into the kitchen sinks, bathroom sink, and bathtub and then try and dump as much water as you can down each drain all at the same time to get as much water flowing into the grey tank as possible.

Hopefully doing this will unclog your grey tank. If they don’t work you can also use the snake method.

See Also: The One And Only Guide To RV Leveling Blocks

Final Thoughts On What To Do If The Black Or Grey Waste Tanks Won’t Drain

Things go wrong with RVs, travel trailers, and 5th-wheels all the time. I’ve had to fix almost everything on my used travel trailer even after just a year of owning it. Having clogged holding/waste tanks is just another part of RV ownership. Hopefully being able to go camping or travel the world in an RV makes up for all of the problems you experience along the way.

There are things you can do to properly maintain RV holding tanks so you never experience any clogs. You can read about that in this article here.

I hope at least one of these methods has worked for you and you haven’t given up on camping or RV life. As a reminder, if you have bought a used RV whether it was from a private seller or a dealer make sure you fill the holding tanks with water and dump your RV before taking it camping. You never know how someone else has treated their RV holding tanks and you can hopefully avoid any future clogs by cleaning your holding tanks really well before using them.

Have any more questions about RV black or grey tanks and fixing waste tank clogs? Leave a comment below.

2 thoughts on “What To Do When An RV Waste Tank (Black/Grey) Won’t Drain”

  1. 2 tips from my RV days….
    1. Always carry spare valves….they only break when you are no where near an RV shop…
    2. I made a sprayer for the tanks from a female hose end epoxyed to a length of PVC sprinkler pie with a 360 degree sprinkler head on it….connected to a hose, I could stick it down through the toilet, or in through the drain pipe and and keep the inside of the tank clean….


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