There's a whole subculture of converted cargo vans, and those who live in them a LOT or full-time swear by the composting toilets. Try www.faroutride.com for a journal of the build and features and comparison tests the owners (both engineers) did for waste disposal in their Transit build.
I still agree with others: in the whole scheme of things, dealing with a black tank isn't all that difficult (says the lady who has toted around a "Luggable Loo" which uses kitty litter and WAG bags for a decade, even with an RV)