Saturday, September 30, 2023


RV’s sinks are plugged but the shower drains, P-traps not clogged. Now what?

Dear Dave,
My RV’s kitchen sinks and bathroom sink are plugged, but the shower drain works fine. The P-traps are not clogged. I sealed the drains with their stoppers, except the main kitchen sink. I then used my shop vac to push air thru the pipes. The result was water came out my gray and black tank vent at the top of my RV. My gray tank stays pretty empty except for the shower water. —Lynn, 2003 Lazy Daze

Dear Lynn,
That is a tough one. The shower typically drains straight down to the gray water tank. The sinks are routed with 2” pipe and some go to the black water tank, depending on the location of the sink and the tank. Other times there are two gray water tanks, again depending on location such as a half-bath in a floorplan.

When you forced air into the kitchen sink, it seems odd that you would get water coming out of the black and gray water tank vents, unless they are somehow plumbed together. I have learned never to say never, or always, when it comes to RV manufacturers!

P-traps not obstructed

Since you verified the P-traps were not obstructed, I would suggest taking them off and run a plumber’s snake down the pipes at both the kitchen and bathroom sink. It may be they are plumbed together and there is an obstruction close to the inlet to the gray water tank.

I just got this response back from Lynn:

Thanks, I will. I just completed the snake and it worked. Had to remove all the plumbing to the last straight piece and it worked. Snaked all the way in to the gray tank. Will flush it well at the next stop. —Lynn

 You might also enjoy this from Dave 

Fresh water drain valve trickles, doesn’t drain

Dear Dave,
I follow the owner’s manual instructions to drain the fresh water tank at the end of a trip. I open both valves in the hold below the bathroom, but I get only a few minutes of drainage. Also, I have tried when changing the fill valve to fill the tank rather than normal use and or opening a faucet. Nothing helps. The only way I can drain the tank is to put the fill valve in normal operation and run the pump. The water then drains from both drain valves. But without the pump running, I cannot drain the tank. Please advise. —Ted, 2010 Winnebago Adventurer 35P

Read Dave’s answer.

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

Read more from Dave here


We have a popular forum for Ask Dave. Please be as brief as possible. Attach a photo or two if it might help Dave with his response. Click to visit Dave’s forum. Or send your inquiries to him using the form below.

Click or drag a file to this area to upload.


Dave Solberg
Dave Solberg
Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.


4.4 9 votes
Article Rating
Subscribe to comments
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments
1 month ago

Would have been good to see a factory plumbing diagram! All gray water drains have “P” traps, or they are supposed to according to code! These “P” traps have several purposes as in its design it keeps water in them to keep out sewer gases & also to catch debris from clogging deeper in the system! They are in both our RV’s & our homes. Would have been helpful to know what was stuck in the drain.

Tom M
1 month ago

Curious about what went down the vanity and kitchen sinks that would block a 2in. drain.

1 month ago

The shower still requires a P trap and depending on how far it is from the gray tank to stop odors and may have some extra plumbing. Only the toilet drains directly into the black tank with no trap.

Sign up for the

RVtravel Newsletter

Sign up and receive 3 FREE RV Checklists: Set-Up, Take-Down and Packing List.