If the toilet holds water, then you do not need a seal. Before you start on any trip on your coach fill up the traps on your sinks, kitchen and bath and run some water through the shower drain, also your washer. It will stop your smell that is venting through the traps.
I would also recommend Happy Camper to eliminate all odors even when dumping. Best thing I have ever used. Do not use the blue stuff they sell at Wal-Marts and some RV stores. That just camouflages the smell. Happy Camper dissolves the solids and turns it back into water. It is an enzyme that eats it up.