Yep, RV/boat toilets are a much more realistic solution, using the flush pedal you put as much water as you need, ideally you want it close to the main drain but worst case you need to do one "big flush" a day so all solids get evacuated.

That's what we've got at our cottage and it's been working flawlessly for years now and the two of us must use less than 10L/day for it. The shower drains in the same pipe so when we take our shower it cleans up any solids that might have clinged...