CamcoGet the largest you can handle and if you've got the room, you might consider 2 of them. Spend the $$ on this one with the bigger wheels because it makes hauling to the dump station easier. It also has an adapter that fits on the front that hooks over the ball of the trailer and you can tow it.
If you do go this route, be aware of the height of the dump valve vs the inlet height of the tote. The only drawback to this particular model is that it is tall. On my trailer, the dump valves are lower than the inlet valve so if the campsite is level, it can be hard to get poop to flow uphill sometimes.
cassette toilet 
