If you're use to tent camping/"roughing it" now, then it shouldn't be too much of a stretch to get a decent cargo trailer and modify it as you go and time/money is available.
Since you don't want to cook or poo inside, really, you're just needing a place to sleep. Should be relatively inexpensive to get a couple of windows put in a cargo trailer, some inexpensive adhesive backed linoleum tile to put down on the floors and a cheap wall covering if you don't like the look of plywood.
Look at the Disc-o-beds for the kids and murphy bed for the adults.
