Black tank odors running AC-vacuum problem?

[Navvets52]

This subject is often brought up, but I can’t find a situation exactly like mine...When running the AC, if I close all vents and windows, there is a strong black tank smell from behind the pocket door between the refrigerator and the toilet room. If I leave the fan vents cracked, sometimes it smells, sometimes it doesn’t. We recently completed a long trip; also when starting from point A to B on that trip, the black tank odors entered the motorhome after first starting out. Any ideas?
2005 Itasca Meridian 34H

[jrod101]

Always have the valves closed outside with cap. Also leave some water on the toilet and should help.

[be happy]

I believe that the AC has nothing to do with your problem as you have a basement AC.

Looking at the floor plan it looks like the wall between the pocket door and the stool contain the black tank vent. Go on the roof and make sure there is a cap on the vent, how close to the 12in square roof vent is the black tank vent. If close and you have the roof vent open you could be getting odor that way. If so you may want to get a different style black tank vent cap. On a different unit I had a problem like that and replaced the vent cap with a Camco Cyclone Rotating Sewer Plumbing Vent. That worked well. You also need to make sure the toilet is holding water in the bowl. If no water then you will get a smell through it also. One other thing if you are on the roof you might as well pull off the cap you have (not hard) and use garden hose to flush the vent. Reason the vent pipe could have become clogged at some time thus not letting the odor escape.
Good Luck

[Topsail]

Originally Posted by Navvets52

This subject is often brought up, but I can’t find a situation exactly like mine...When running the AC, if I close all vents and windows, there is a strong black tank smell from behind the pocket door between the refrigerator and the toilet room. If I leave the fan vents cracked, sometimes it smells, sometimes it doesn’t. We recently completed a long trip; also when starting from point A to B on that trip, the black tank odors entered the motorhome after first starting out. Any ideas?
2005 Itasca Meridian 34H

When you run the AC you are drawing a vacuum inside the coach. You may have a poor connection coming off of the black tank vent stack where it connects onto the black tank. This seems to come up on other Winnebagos. Hopefully you can gain access to verify this. Maybe buy an inexpensive camera from someplace like Harbor Feight that has the extension on a probe. Let us know when you figure this out.

[Morich]

Another potential "stink" leaker is sometimes easier to find but hard to test.
Near each sin or vanity there will be a water trap which should always have water in the bottom to act as an oder seal to keep the smell from coming up the drain into the room. In houses these are vented to the roof in most cases but in RV there is an anti-siphon valve used. If these valves/flaps get fouled and don't fully close each time, the smell can come out of them or the water may not stay in the trap as it should.
You have three of these so a check may be needed.
First try to see down the trap to see if water is actually staying in them. During storage these can evaporate dry but if the anti-siphon doesn't let air in the water going down the line can actually suck this water out and leave the trap open.
These are sometimes hidden but often left accessible under the vanity and sink where we can spot them.
On your year, make, model the info is not as precise as some but the attached may help find them? Click the picture to get a larger /better view but valves are marked blue and traps green.
Sometimes the only way to find if the valve is working is to change it!!!

[Just Mike]

We bought a 2019 2108DS used this year, and there was a slight odor from the black tank. I replaced the flush ball seal in our Dometic toilet and that helped seal the opening between the toilet bowl and the tank. This eliminated the odor, and might be the same thing you are dealing with.

[Morich]

Originally Posted by Just Mike

We bought a 2019 2108DS used this year, and there was a slight odor from the black tank. I replaced the flush ball seal in our Dometic toilet and that helped seal the opening between the toilet bowl and the tank. This eliminated the odor, and might be the same thing you are dealing with.

Very GOOD point!
This is a little point that can be easy to miss but make big issues to solve.
We have become more aware of this over time and it is super simple to test if we think about it.
We want the stool seal to be tight and good enough to keep odor from coming up into the RV and one way to do this and make it somewhat obvious when it fails is in the the adjustment of the stool.
If we don't want odor to come up it has to seal well enough to not allow water to go down! There are often adjustments on the way this seal and the water work, so it can be a good clue if we adjust the stool to close the seal a couple seconds before the water stop flowing as we flush. This leaves a thin layer of water on top of the seal and that lets us do an easy check of the seal working right!
As long as the stool winds up with a thin layer of water on top of the seal and it stays there, we know the seal is working right.
Maybe worth doing a quick look next rip out?