A mate of mine linked this on Facebook the other day. It's of a bloke showing us round his converted Bedford lorry which now houses himself and his partner. Anyone ever seen a composting toilet in a caravan/campervan before?
I have used a sawdust toilet in a camper van. It dries stuff out just like kitty litter. I occasionally buried the contents. Much better than a chemical toilet. 2 five gallon pails. One use the toilet, the other holds sawdust. Dry soil can be used instead of or mixed with sawdust.
Pee in a jar for best results. A good system for anyone who has enemies.