They make "smoke Bombs" for testing plumbing systems. The smoke is odorless and harmless. I used them as a Water and Sewer Commissioner to identify those that have tied their rainwater systems into the waste sewer system and it also freaked out home owners that had "untrapped" lines in their home. The answer to the outrage of those that had smoke in their house was that it identified a problem that could actually kill the residents or make them sick and that we had found a plumbing code violation caused by the builder. Keeping children in that environment was a criminal offense, or could be construed as such. Once it was "explained" we got a big apology and assurances that a plumber would correct the problem the next day. That was an elected position but was close to a Gummint Worker and I guess made me a politician of sorts. The sanitation workers did the job and I was there to answer to the "voters" and, you know, sugar coat everything.
This is a problem that baffles me no end. It has a solution within the bounds of accepted and common practice because code and Winnebago would not have any truck with a failed process/practice. I have never heard anyone, particularly myself, say that venting down would not work. It is a design that many here are using to their great and odor free satisfaction. I refuse to argue with another's success. But, by Yiminey, it ain't code and it ain't used by builders and that has always been my questioning point. WHY?
I was told that you can have "NO" bends or turns in the vent and that it must go straight up. Well hooie! My ancient Winnie has two 45 degree fittings in the black water vent that runs up and thru the roof via next to the toilet. All others are arrow straight. The black tank has two vents if memory serves. To me the system function is obvious. While sitting, the gasses will go up the pipe that presents the least resistance to their flow. By default, the air intake to the tank to replace the vented air will come down the second vent pipe. When I fill the toilet with water and give it a "quick" flush, I momentarily pressurize the tank and a gush of air goes up through the vents. Never went up there to check. The inertia of that air flow is what causes the air to go into the toilet when the water is gone down. I can open the toilet and use a flashlight to see how full the little Darlin actually is. With my head down in the toilet, figuratively speaking, I smell nothing wrong regardless of how full the tank might be. NADA!
I have vent caps on my vents........sometimes. They blow off and I religiously replace them within 6 months like clockwork.
My black tank has been sitting open for 3 months. I mean there is a BIG hole in the floor and it goes into the tank....directly drops straight down. Ker plunk!. Empty now but has been opened like that in the past. NO ODER. Design success? I think so. Can you achieve this level of performance with another design? Apparently.
Your bathroom door will seal batroom odors in the bathroom only if said door is of the hermetically sealing variety. (prepare to repell boarders!)
I once had a severe, drive you out of the house, GOD AWFUL, toilet odor. It didn't used to be and then it was. After flushing the toilet, well, you cannot even imagine. I replaced the toilet seal a few times. i doubled up the toilet seal. I caulked and glued the screw in fitting to the toilet flange tank connection. Then I set the flange in silicone. I even pulled the flange and tiled the floor "under" the toilet so that I could raise the flange the thickness of the tile on the floor. I put vent covers on and I took vent covers off. It never left the park while I rode this Merry Go Round to nowhere. I lived with this situation for a month and a half and I lost a lot of weight. My appetite was curbed even to go out and eat Chinese in a restaurant. Remember that 45 degree bend I mentioned a while back? The one in the vent stack that was behind the toilet? The one someone might possibly use as a hand rail under some unforeseen nor recorded circumstance? That pipe had a crummy crack in it on the back and under side below the 45 fitting. The weight of the vent stack kept the crack well open and the toilet gas vented into the coach. Hack sawed out the section of ABS and glued in a replacement piece. Sat in the corner and recited the "Little Jack Horner" rhyme. Not really!
I made that part up.
When people started talking about smoke I reflected that my odor problem was so severe that I would have believed it was coming from under the bed if that is what i was told. I couldn't find it by smelling around as the gas was going up the vent and the crack w3as apparently a point where gas entered unless a flush was about.
I have never had odors come up thru the traps. Think about the shape of the front of an ancient Winnie. The ones that looked like a Neanderthal had a hand in the RV's design. Remember that old girl? Well, if you open the drivers window while going down the road at 60 the outflow will nearly suck you out that window if you have other windows open in the rear. I kept the drivers slider open a bare 1/4 of an inch and my cigarette smoke never ever got inside. The amt of suck in that coach was worthy of the film industry BUT it never pulled gas up thru the P trap or toilet. Never! If you have a conventional P trap and there is water in it look elsewhere for the problem. But that odor will drive you to great and unnecessary lengths. You have my sympathy.
Hope this helped, hope it made you smile a little. After all, others misery is always a yuk.