I did take a close look at adding a Nature's Head composting toilet, a tankless water heater, and a 12v compressor refrigerator
to an Oliver
. Decided against the Nature's Head toilet after talking this over with Robert Partee of Oliver. For the composting toilet you do want to install a vent pipe and it would be best to use 12v to run a small exhaust fan on that vent pipe. Oliver is very careful/concerned with added hull penetrations. Per Robert, their vent pipe for holding tanks is molded into the hull and would be difficult to make connections for the toilet vent. He also brought up the concern of height as the existing toilet sits on a step. With the Nature's Head installed on the step you would sit but your feet would probably dangle. We did not get to the 12v fan issue as the vent pipe connection decided against installation.
I do believe the composting toilet is a very smart idea but probably better suited for a flat floor, dry bath installation. The discussion and videos made by Gone with the Wynns concerning composting toilet usage in their motor homes is very good. They installed one in their old motor home and liked it so well they took it with them when they upgraded to their new motor home.