Tuesday, November 28, 2023


Ask Dave: Why is there a sewer smell in coach after leaving a campground?

Dear Dave,
I have a 2010 Winnebago Adventurer 35P. When we start traveling after using our coach in a campground we get a horrible sewer smell throughout the entire coach. We open some windows and the smell goes away in about 20 minutes. I have replaced the vent covers under both sinks, and I make sure there is water in all the traps. It happens on empty, partially full or full tanks. The shower vent pipe has a cyclone vent cap on the roof. We get no sewer smell when parked. What should I look for next? —Ted

Dear Ted,
The two under-sink vents you replaced we call “cheater vents” and are for the gray water. They should not produce a sewer-like smell. Here is the drawing from Winnebago’s website on those vents.

Might need to sanitize the gray tank

Since they are under a counter, the vent pipe cannot go all the way to the top and out the roof. so they use a “syphon cap”—which I am not a big fan of. Again, since this is the gray water that has the shower drain, it should not have a sewage smell. However, you might try sanitizing the gray water tank just in case. I have seen a few units that had a terrible smell from the gray water that sat for a long period of time with grease from the kitchen sink.

Here is the diagram of the toilet and vent pipes that go to the top of your unit.

There are grommets and couplers that could have come loose or even cracked at the top of the tank. These vent pipes are most likely hidden in the bathroom wall. You might have to take the paneling off to access and inspect them. If they are cracked, or just came loose, the cyclone vent will actually draw the odors out of the tank and they will escape inside the rig.

The smell could be from the toilet flange

Another issue could be the toilet flange or grommet, as seen in the exploded circle of the above diagram. Take off the plastic base and make sure everything is connected firmly. Check if there is any odor coming from this.

This should all be a closed system. Therefore, if there is an odor coming inside, I would believe there is an open area in one of the vent lines and the cyclone is pulling it into the coach. Another thing you could try is to have the tank empty, close off the cyclone vents and test drive down the road. If there is not a smell, you have your answer.

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.

Read more from Dave here


We have started a new forum link 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 Solberghttp://www.rv-seminars.com/
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.



0 0 votes
Article Rating
Subscribe to comments
Notify of

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

Oldest Most Voted
Inline Feedbacks
View all comments

Zack (@guest_173065)
1 year ago

I capped those cheater vents in both bathroom and kitchen sinks 4 years ago due to same issue. I’m not stressing a code violation in my coach. No affect on drainage and the smell is gone. Save yourself some headaches and spend $5 on 2 caps and eliminate that variable. I agree if that doesn’t help, then a crack in another vent or toilet seal likely exists. You got this!

Steve (@guest_172893)
1 year ago

You can also use a check valve to prevent sewer odors. P Trap Pal is a simple to install plumbing check valve that can be installed by hand with no tools (www.ptrappal.com). This may be a solution for your sewer odor issues.

Drew (@guest_172709)
1 year ago

There’s another type of cheater vent cap that is higher quality than most you buy at the hdwe store. Here’s one from Lowe’s:


Paul (@guest_172706)
1 year ago

Note on the above diagrams that the lavatory sink is plumbed to the black tank along with the commode. This is true of most Winnebago rigs to ensure that you get sufficient liquid into the black tank. So the comment that the both under sink vents go to the gray tank is not entirely so. I wonder too if there is a washer/ dryer installed. If not, the plumbing installed at the factory for that option may have an open & empty trap that could be the source of odors from the gray tank. Another odor source on my Winnebago can be the rear roof vent, which is not far from the single roof vent for the holding tanks. If it is open while driving, it will often pull in odors from the tank vent.

Jesse Crouse (@guest_172656)
1 year ago

I agree with Dave about the “cheater” vents. I think all vents should go up and out the roof or at least tied into a loop vent system. Gray should be seperate from the black tank system.

Sign up for the

RVtravel Newsletter

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