I added a Thetford C4 manual flush cassette under the front bench of my Aliner
clicky. It was more accessible inside the camper to install it in the center of the trailer but this location requires a longer reach through the access door to empty the cassette
(not really all that difficult). I bought my Aliner used and the toilet was a relatively easy modification. The manual flush has the advantage of being self-contained not needing any additional plumbing or wiring.