water inside camper when using black tank flush

AuntNess · October 2025

I have a 2018 Tab 400. On a recent trip I used the black tank flush and while water was turned on, I stepped into camper and there was water on floor. I ran to turn water off and then quickly threw a towel down. I couldn't tell where it was coming from: under shower pan or closet. How do I check line coming in from outside? I think I may have to remove bathroom sink to see. Any advice or helpful info would be appreciated.

qhumberd · October 2025

On your black tank flush there is a check valve behind the sink that could be a leak point but the only way to check it is to remove the folding sink.

Here is how I took mine down with pictures from my 2019, which should be similar

https://tab-rv.vanillacommunity.com/discussion/15361/t-b-400-bathroom-folding-sink-removal-add-panels-and-shelf#latest

The valve pictures are here (white lines going behind panel going to check valve and then down to black tank):

Image: https://us.v-cdn.net/5021717/uploads/editor/hf/nghg2lhcrepk.jpg

Image: https://us.v-cdn.net/5021717/uploads/editor/em/dvcpi7nfe64m.jpg

Image: https://us.v-cdn.net/5021717/uploads/editor/jq/b8hsobj8ifbd.jpg

AuntNess · October 2025

Thank you for the information and the photos. This helps a lot!

qhumberd · October 2025

@AuntNess others have mentioned a small borescope to get a look without complete teardown of the area. If you can see a connection or that check valve leaking when you have eyes on, that would be an easy fix. As you can see from the pics the line goes from the outside of the trailer up to the check valve and then down to the black tank so your options for leaking are contained at least. I have no idea how to access the entry point to the black tank, but think that might not get water on the bath floor since it is below that leve.

Let us know what you learn.

subkron · October 2025

I had a similar issue, found out it was a (dead) stick bug that got into the valve that @qhumberd had detailed above and kept the valve from fully closing. I cleaned it out and put some screening around the valve.

water inside camper when using black tank flush

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