We had a cassette toilet in one of our pop-up campers, where it was built in, and all you could remove is the tank to empty it.  We actually had two, as one you removed the tank from inside the trailer, and the other had a door outside to pull the tank out (that was the best way.  The last one had a that wood made out of wood scraps glued together, and it drew moisture in the cold as the cabinet was colder than the main area, and it became swollen, and a disaster.  My preference from here on out would be the standard porti-potti.  I liked it better because I could hose the thing down, as all of those plastic toilets have lots of crevices, and I just could not get the cassette toilet clean enough to suit me.  Maybe look at photos of the Lil' Snoozy, as they build one into the toilet closet, water comes in automatically and I think you can pull the tank out to empty it from outside, or at least that is the way they used to be.  Actually, just checked their video, and they use a Thedford Model 223.