Here is an economical alternative: Amazon.com: Reliance Products Luggable Loo Portable 5 Gallon Toilet: Sports & Outdoors
People use Poo Powder with sawdust or cat litter etc. with the heavy duty trash bags. Tie it up and toss when done. We only use it for liquids unless there is an emergency which so far there hasn't been. No pouring, sloshing etc. Neat and tidy and no odor at least not with good cat litter. The bucket height is perfect no adding a base to raise it etc. The only downside is it wont fit in the cubby under the front bunks but I use it for other storage instead anyway.