Here are some
Thetford toilets from PPL & prices.
Here are
Sealand toilets same PPL.
All my coach's have had the porcelain Sealands they have been trouble free and easy to change out or repair.
When I say that the toilets never needed a repair it was a soil pipe connected to floor flange that dropped out of flange into black tank, it was never glued into flange.
They are easy to keep clean and come with different size bowls and always have had a good seal to keep the water in bowl.
Your foam seal has to lay flat over flange and raised base placed flat over the seal, the Sealand base detaches from bowl so its easy to place base support on seal.
If user is heavy set there is a 4 bolt base with adapter for a more solid support.