We've been plagued by toilet odor since buying our new 21FBRS last November. Even a return to the dealer didn't fix it. Today I decided to pull the toilet (Dometic model 300) myself to see if the floor seal was bad. The floor seal looked fine, but when I turned the toilet over, sewage came out.

It was trapped between the bowl and the outer housing, just as you found. Placing it upside down, I filled the discharge pipe with water and watched as it leaked out of the middle where the two pieces of housing are joined. Bad gasket -- the one which is
not user replaceable. Returned it to the dealer today. Hopefully will get a replacement tomorrow.