Tuesday, March 21, 2023


RV’s underbelly filling up with water. How do I find the source?

Dear Dave,
We may have a leak in the kitchen’s gray tank as the underbelly fills with water and it stinks up the whole place. We empty the tank every two days (used to be four) as I wash a lot of fruit and veggies. Lately we just leave it open. We drilled a couple of holes in the underbelly, about where the middle jacks are (under the office/bunk house slide area), to let it drain faster.

We don’t want to pull down the underbelly. As you know, there are too many things connected to it that would be a huge pain to remove. We got an endoscope for Christmas, but, as sweet as it is, the camera is not easy to maneuver inside the underbelly.

What is the easiest way to find out where the leak is? We have in the past tried a few times to get Grand Design to send us a blueprint of our RV but they never did. Because we full-time we don’t really want to take it into a dealership either. We don’t think it’s the black tank because the smell clears after draining the kitchen tank, and the shower tank is always open because of the washer. Thanks! —L. Beal, 2019 Grand Design Reflection 367BHS

Dear L. Beal,
The first thing I would do is a test to make sure whether it is the gray or black water tank. Put some blue food coloring down the shower drain and run some water. If the water you are getting in the underbelly is blue, you know it is that tank. You might have more than one gray water tank, so this would also isolate the tank. If it is not blue, it’s obviously not that tank and you have more of a mess!

Once you isolate the tank, use your endoscope and connect it to a fiberglass “fish” pole like this one.

Use these green fiberglass fish poles to check for leaks in a gray tank

You can tape the endoscope to this and should have a light on your scope. The rigid poles are much easier to control the scope and you can add lengths as you go.

Pay attention to the time to determine leak in gray tank

To make it easier to identify where the leak is coming from, pay attention to when you first see water in the underbelly. Empty the tank and then calculate when you see water. If it is fairly soon after it starts filling, it is at the bottom of the tank and could be at the drain valve connection. These are typically spun into the side of the tank and could have a pinhole or be cracked.

If it is a longer time after the tank was emptied, then the leak will be higher up as the water level rises. If this is the case, it is most likely a crack in the edge of the tank. Most tanks are secured in the compartment with either wood or metal braces and the tank can rub against them and create a leak.

If it doesn’t start to leak out of the gray tank until it is almost full, then it is most likely the vent connection. Most likely it would be the vent pipe that is designed to go up to the roof or that a cheater vent has dislodged from the connection and the water is splashing out the top and running down the sides.


As for the underbelly, if you can isolate it somewhat you don’t need to take the entire bottom off. Just cut a square large enough to access the spot to see if you can repair it with a plastic weld kit. Cut the material just short of the cross members it is secured to and buy some new material, cut it slightly larger than the hole and overlap it. You can secure the new material to the same brackets and either seal it with an appropriate sealant or use Eternabond.

 You might also enjoy this

That stink in your RV is probably the unsuspected gray water tank

By Nanci Dixon
The tech kept telling me, “That might just be the gray water tank. It can smell as bad as the black.” Actually, at least three or four techs told me that every time I brought up how BAD it smells inside the motorhome when we are traveling. Three of them said to add Pine Power to the tank. Continue reading.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here


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Gwynn W
1 month ago

Thank you for this information. I’m having the same issue with my 2020 Grand Design Imagine. According to my research this is a common problem with specific year models.

Thomas D
1 month ago

I needed to get underneath to a tank and used a razor knife to cut away the coroplast underbelly. Cut on 3 sides and after repairs I used black ty wraps to stitch it back together. Its been years and they are still working. Just push through with razor make a 1/2 ” slkt, install tywrap and when you feel you have enough pull them tight.cut off excess

Tom E
1 month ago

Bought our 2016 Coachmen Chaparral 5th wheel used in the winter of 2020 in Florida from a fellow snowbirder. A week or so after hooking up the water I noticed a small drip coming from below the under belly and the coroplast was bowing down. Previous (Michigan) owner said nothing about a leak but did say the black tank valve was stuck open (nope, not the source – that I replaced later). After tearing out everything in the cargo compartment I discovered the water pump housing was cracked and there was a slow drip from it – so slow that it took a week to collect enough water to start dripping below. So if you’ve stored the RV in freezing conditions, one thing to check for a leak would be the water pump housing. I replaced just the housing (for 1/4th the cost of a pump) and it eliminated the water leak. I now use the water pump to fill my water lines with RV antifreeze, not simply blow out the lines with air.

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