Have you considered a cassette toilet? These are similar in concept to high-end portables, except that what you take to dump is a removable tank, the cassette. Some want a water supply, others have their own supply tank (to be filled at time of dump).
Composting toilets often need separate disposal paths for liquid waste. In permanent locations, urine is often simple piped to where it dumps on the ground, or else to a separate tank for later disposal. I'm not sure how this fits into a mobile situation, and might be why mobile composters get expensive.
