I can't really comment on what to do yet as I've not gotten that far and I plan to fully engineer a custom solution and make use of a full sized house flushing toilet.
It is my belief that the throne isn't something you should compromise on. All one would need to do is put a water hose to the toilet so it can re-fill the bowl, and a flush drain to a black water tank with a vent pipe to help flow.
Given these requirements, the technicals of installation will work quite easily.
The challenge then comes with how do you keep the water in the bowl from sloshing and spilling out. And that is also a simple modification.
1.) The toilet bowl lid as well as the top cover can all be used, you just simply have to get some rubber trim and seal the top cover lid, and install 2 clamp locks to apply some additional pressure to lock the top lid down and prevent water from spilling out the seems.
2.) For the lid same thing, rubber gasket trim, and put a clamp style lock on the front of the lid.
Now will this prevent noise and possible sloshing while driving, no. Is it something I can live with, yes because nobody will be in the back while driving the bus, they'll be up front on the couch seats with seatbelts, and won't hear it. I can also add some baffles inside of the upper toilet bowl to help with sloshing. And as long as it doesn't spill out I don't care.
The only negative to this is I might need a larger water tank as I'm using water to flush with, but where I live water is plentiful and practically free, and we'll try to use public bathrooms when we can to save on water. Also using a full flushing toilet, i don't have to worry about fans. If my wife decides to drop a bomb and stinks up the bus, I will have dual large ceiling fans in the back and front of bus to quickly swap out all of the air in the bus. Will only take a couple of seconds to do so.