Sewer smell in bathroom

I have a Dometic 300 toilet. We got a persistent sewer smell and I could not figure out where it was coming from. In desperation, I removed the toilet from the trailer, and voila, no more smell in the trailer. I disassembled the toilet and found that it has an inner and an outer shell, with an "O" ring between them. In my case, the "O" ring, which has a square cross-section, was rolled over so that it didn't seal. I found about half a cupful of black water in between the two shells.

I would recommend you do all the things others have suggested first. Then, if the smell persists and you have a similar toilet to mine, you just might remove it and investigate whether there is black water trapped somewhere.