Black tank smell.

Your bowl is holding water so the seal on the flush mechanism is good BUT, there is a foam seal between the bottom of your toilet and the flange on the floor. These always go bad eventually as the foam dries out. When there is a black tank odor problem and the bowl is holding water, it is almost always the foam seal.

When you have the toilet off to replace the seal, you can inspect the pipe and flange for any cracks but there hardly ever is. If that's all good for some reason and there is still a smell, you need to find your vent pipe and start looking/smelling for leaks.

No matter what the smell in your tanks, you should not smell it in the RV. Dumping treatments and cleaning the tank is not addressing the true issue.

Thanks for the tips wolfe10. I'll check those things.

With your description, I agree you need to take a close look at your roof-top vent.

It could have fallen down into the tank so it doesn't service as a vent, particularly at higher tank levels.

Could have an insect nest blocking it.

If it passes those two tests, I would install one of the venturi vent covers.

The object is to make sure that there is more pressure in the coach than in the tank.

I have the same problem as the OP, but only when I flush. I use the geo method, as well as clean out the tank when I return home with pressure wand down the toilet. I have read all your answers and have a few further questions.

I have a split bathroom, toilet on one side, sink and shower on the other. When you talk about the check valve, flapper, p-traps under the sink.....if there was a problem under the sink, would it smell right there? Because I stuck my head in the cabinet under the sink and it smells fresh as a daisy.

If I am using the motorhome, which p-trap could be dry? I do have a black tank flush system that I don't use.

Since this happens only when I flush and my toilet holds water, I believe all my toilet seals are good.

How about the standpipe vent on the roof. What exactly would I do in checking that? Do I put a plumber's snake down it? Could the protective cover actually be pushed on too tight, thereby blocking air from getting out?

Thanks for any insight you can give me.

Put 3 inches of water in and 10-20 lb of ice and drive around for 1/2 hr then drain and addd 3 inches of water. Odor should go away. We have never had an odor problem using this method.

You might be surprised the grey tank can smell as bad as the black! So make sure water in the shower trap, between lack of use and shaking going down the road they empty. Also if your coach is plumbed for a washer make sure the cover is on if no washer. If you have a washer make sure there is water in that trap. Those are the only additions I have, we went to the Cyclone vent covers and the only issue we see is empty traps now.

I've never seen a reason to use chems in our motorhome or our 5th wheel before that. When we DO notice an odor, we know we've got a dry trap (most common) or a bad vent valve.

Jim

All of these comments are excellent .... go one by one and you should come up with an answer ..... my only additional is: maybe it's the coach next to you and your down wind!!
:B

That solarvent looks interesting...

Since you are fulltimer, this likely won't fit your situation. What we found is we would put in tank treatment right after dumping on way home from a weekend camping. It would be several weeks before next trip and the treatment seemed to wear off. Now, we don't add treatment until day we head out. The driving seems to stir it around and cut odor.

Another possible issue is if water in your toilet bowl leaks down or evaporates from long term storage. You need a little water left in bowl to seal off odors.

I store the sewer dump hose in the compartment (under the black tank) where the outlet is located. The hose itself will retain orders and pass them along to the closed compartment which has openings up to under the bathroom sink. To control this hose odor, I soak the hose and fittings in "bleach water" in a plastic tub at least once a year.

Under my bathroom sink are two vents, one for the sink drain and one which is a backflow preventer on the black tank flush system. Just to be sure, I covered both with loosely placed plastic shopping bags cinched tight around the pipe with a cable tie. The loosely placed bags allow the vents to work, but help prevent any odors.

Pictured below is a replacement solar powered roof vent that might help:

From this web site:Solar Powered RV Plumbing Vent