George,
The answer to that question will depend upon how your coach is equipped. If your coach has the standard gravity flush Sealand toilet, you can use any toilet seat, but you should use one with good cushions where it contacts the bowl to prevent wear from roaf vibrations. However, if you have a Vacuflush toilet a standard seat will not have the rubber sealing ring between the seat and bowl that the original seat used. I personally do not think that this seal is necessary, but many owners do, so it is up to you, but a cushion where the seat contacts the bowl is advise able.
Gerald