What's it going to be riding on; the Uni? Weight is also a factor. You say 10 ft long; how wide is it?

What kind of shape is the roof in? You might be able to suspend 2 bunks that fold out of the way on piano hinges. Bottom bunks can have some storage for bedding when the upper bunks are stowed beneath them, again either with a chest style or pullout drawer.

Our local Fred Meyer sells 3' x 6' camper foam pads during the summer for that kind of thing. You can make slip covers for them for next to nothing to keep 'em nice and to pull off for cleaning once in awhile. I've got my dog's floor pillows set up like that.

RV supply stores/sites are probably your best bet for modular 12v and solar goodies. The cabinets you can make.

If thinking also about a privy, I'd just get a camp toilet you can set outside for use, and service it at a pit toilet. Ours came in handy for while I had the sewer dug up a few years back. You can get one at a good hardware or sporting goods store, along with some blue toilet chemical to keep the smell at bay for a nominal price.