That's what I would like to build out someday as well.
Some of the build-outs are fairly fancy, but I just want a small counter, cassette toilet, and a way to put bunks in.
For a sink, I wouldn't bother with tanks. There's some good ideas online that show how to use portable water containers, either mounted up high so the water flow is via gravity, or below and pump-driven. Grey water could be collected in a container outside. I wouldn't bother with a shower either; we never use the one in our travel trailer. Maybe an outside shower setup, with a way to hook up a curtain on the trailer's outer wall, I suppose.
I may also put in a
HappiJac bed lift, depending on whether IncrediGirl wants to come along too.
Then I wouldn't need my travel trailer at all.
For heating, a basic direct vent LP system would work for me. Not sure if I would get one of those trendy Newport ones, but one of those Ashley's should work. I'm trying to keep away from needing batteries to run heat with, plus the sound of a furnace fan kicking in and out all of the time can be annoying when you're trying to sleep. And, if space allows, a little wood stove would be awesome.