[sourdough]

A Dometic 310/320 with soft close wood lid would fit the bill for you I would think unless you just want the flush handle on the tank. What you're proposing is not only going to be more work than you think for just the toilet installation but the ramifications for flushing, black waste storage etc. become problems. If you choose to use a res toilet I would pull that black tank and run residential sewer pipe. I've seen it done; it's not pretty and certainly not mobile. It does however eliminate dumping the black tank.

Do not leave the black tank drain open and then run res sewer pipe. DS bought a nice trailer that had been full timed in set up like that and it worked for a couple of weeks - then he started calling me because his black water kept backing up. After considerable $$ and work he finally got the poo pyramid and dried debris in the sewer pipe cleaned out and the black tank functioning again...working fine keeping the drain valve closed.