Sunday, October 2, 2022


RVer suspects sewage leak, but where?

By Chris Dougherty
Dear Chris,
toilet leakI suspect we have a leak in our black water system between the toilet and tank, which sits directly below the toilet. The pipe goes straight down to the tank and it appears that this pipe terminates about an inch or so from the bottom of the tank. I cannot tell how this pipe is attached to the toilet, but that is where I suspect the leak may be. I assume that this is the common setup for black water systems and wanted to see if there is a schematic or other information on just how the parts come together. —Dave

Dear Dave,
Each manufacturer assembles its black water system slightly differently. Some use threaded connections at the toilet flange, which also gets screwed to the floor. and the tank. Other manufacturers glue the pieces together.

There are several openings in the tank that can leak. If you suspect the leak is at the top of the tank, there are two connections. One is for the 3″ pipe coming from the toilet, and there’s a 1.5″ connection for the roof vent.

When the tank comes from the manufacturer it is blank, or has no openings or fittings attached. A hole saw is used to cut the holes, and fittings are then glued in place. The pipe is either screwed in or glued into the fitting. I have seen the tanks and fittings break at this point numerous times. If the tank only leaks when it is completely full, then this is likely where the leak is.

Chris Dougherty is a certified RV technician. He received the above from a reader while he was serving as’s technical editor.


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Clayton D
2 months ago

Most black/gray water tanks are shipped from the factory nested meaning no tops on each tank. The tops are attached when installed as well as holes being cut in the proper places. Many times the top will separate (come unglued) in small sections. As a result, when the tank is full or you are traveling you will experience seepage with the telltale smell. This has occurred on 2 different gray water tanks on our 5th wheel.

Rick Myers
8 months ago

My 5th wheel has a rubber “O”ring where the 3” pipe goes into the tank. I had a commercial septic truck suck out my black tank, he cranked up the truck to develop maximum vacuum, I heard a pop, didn’t find out what popped until after several times while coming to a stop and we could smell sewer. I found that the suction from the septic truck had pulled the “O” ring partially into the tank creating a leak at the top of the tank. I removed toilet and 3″ pipe then replaced the “O” ring. Next time (if there is a next time) I will open the flush valve on the toilet when it is being pumped out.

Jesse W Crouse
1 year ago

Just a suggestion from a plumber. We had a leak at the base of the toilet and it was just a neoprene gasket that needed to be replaced. Just like your toilet at home.

1 year ago

My first question would have been “Do you have a Dometic model 300 toilet?”

Those are notorious for trapping waste in their innards and smelling the place up.

2 months ago
Reply to  J J

Happened to our TT. Replaced the toilet and got rid of the outhouse smell.

Jerry W
1 year ago

I had a leak between the toilet and the tank too. It was caused when the strap that held the black tank in place broke where the single screw was holding it. The black tank dropped a bit allowing sewage to escape. The pipe between the toilet and tank was neither screwed in or glued. The gap was too small to see looking down the toilet but large enough to create a mess. 

1 year ago

My black water tank leaked in our first motorhome, right at the exit of the tank (#11 in diagram above). A hairline crack, about 3/4″, at the 3″ pipe/collar connection. There’s lots of strain at this joint from road vibration. Every patch I put on it worked for a while, until I exceeded 3″ of black mess in the tank. Then the seep would start, out that little repaired crack. Worse when I drove the motorhome. I used maybe ten different repair methods, over a two year period, glues, patches and epoxys. Urine is corrosive and after a week, all glues separated. Period. I understand the only solution was to find a plastic welder to repair the ABS leak. I never did that and traded it in.

1 year ago

Is is possible the leak could be from the water line in the back?

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