What would you do with the waste water from the shower or a sink?
There are composting toilets that evaporate fluids and break solids down into something like potting soil. A web search for "compost toilet" should yield a lot of info.
Incinerator toilets essentially do the same, but they can be expensive to purchase, to fuel and to maintain, and they can generate quite a bit of heat.
If you end up going elsewhere to shower and so on anyway, there are RV-style "suitcase toilets" that can be carried away and dumped in any conventional toilet ... and my wife and I once stayed with a friend where we had nothing more than our own 5-gallon pail lined with a garbage bag and covered with a lid. In a case like that, sawdust is great for reducing odor and slosh, and Cabela's sells a snap-on toilet seat.