We have the Thetford "PortaPotty" 135 that was included with our boat. (see Thetford.com)
It works fine for its intended purpose - after we actually read the instructions on how to flush it, we haven't had any problems :mellow: It is a two-step process to flush (pull out a handle, then pump a bellow) which were not intuitive so my wife wrote the instructions with permenant marker on the lid!
It is not very tall so I built a box out of plywood and 2x4s for it to sit on - it positions it at a more "normal" toilet height
It is not really (or very gross) to empty either. Again, once we read the instructions, it all functions adequately. Bottom line, it is better than just going over the side which I guess some people find disconcerting :lol:
Have added a picture - black strap across it to keep it from falling of the box which it did on my first trip with it. It ended up upside down however nothing leaked out. Also the blue capped thing underneath it is a container my wife found at a Dollar General store - use it to hold tp.