The Fiamma Bi-Pot is my loo of choice. They come from europe and are made of two nealy identical 5 gallon containers. Both are easy to carry, dump and fill and it is high enough to feel just like home. We have bought three, two for previous RV's and one for the house for emergencies and occesional sickroom use.
Available from J.C. Whitey and other RV suppliers.
Here's a link to Fiamma's site, scroll down to the dwg for a better idea.