We have a 1980's Thetford Aqua Magic V (high) foot flush in the Class C. It is a comfortable height to sit down on. The "low" ones are designed to sit on a raised ledge. We have a used "low" one that we are going to canabalize for the water valve (expensive gizmo... also cheaper to buy a new toilet). We also like the foot flush models over the hand flush. Only problem we have found is both my daughter and I forget and try to foot flush a home style toilet.
One of the better prices... don't forget to add in the shipping when you start pricing...
http://cgi.ebay.com/ebaymotors/Thetford ... vi-content
For the skoolie, we pulled a vintage porcelain Mansfield Traveler 910 (now sold as a Sealand Traveler 910) out of my mom's travel trailer a few years back and stored it. Nice design.