We also have a 2011 Puma trailer with a Thetford Aqua Magic V. We began having the same issue with smell coming into the bathroom (got worse when you sat on the seat). What was even more interesting is that the toilet holds water indefinitely, so I knew the flush plate and seal were fine. Oddly enough, after dealing with the smell during our last camping trip, I made up my mind that I was going to get to the bottom of this when we got home. Before we left the campsite though, I gave the whole inside (underside) of the toilet a good scrub and spray so it would be clean to work on. Interestingly enough, I noticed water on the floor behind the toilet when I was done cleaning it (and I know for certain that it didn't come from the outside). I was convinced that it was the bowl-to-floor seal. After removing the toilet from the floor, I decided to clean it even further now that I had access to it from the bottom. With the toilet now upside down, I filled it with a bit of soap and water to give it a good scrubbing - and THAT'S when the problem reared its head. The water was coming out of the seam at the top of the toilet where the base of the toilet is fitted to the top of the toilet. In earlier models, this was assembled with screws and easy to disassemble. The seal between the two was just a clay duct-seal style of material. With this model though, they decided to permanently bond the two together. This is all fine and dandy unless it's a poor bond and the two parts start to separate! This explained why the gases were worse when sitting on the toilet as well as the water on the floor after cleaning it. My solution was very simple. With the whole toilet now super clean and dry, I used a product made by DAP called Ultra Clear. It's a flexible sealant that dries crystal clear and is immediate water ready. It's the best stuff I've ever seen! I simply ran a bead of this along the seal making sure that it fills the joint between the two pieces. Although the floor seal looked good, I replaced it with a new one anyway. Before I put the new floor seal on the bottom of the toilet, I covered the WHOLE seal with silicone grease (must be silicone as anything petroleum-based will deteriorate the seal). Do not use silicone spray, you want silicone grease as it will stay put and create an absolutely water and gas-tight seal where the toilet meets the floor. This same grease is also perfect for lubricating the flush plate and seal! It's been several trips since I had done this and we've never had so much as a whiff ever again. Hope this helps you out in your endeavor to rid your RV of this common problem.
