I've seen a handicap portapotty out at the lake that is about that size... a few modifications and you'd have it.
Truth is... many people just use a camper shell on their truck and a sleeping bag.
We elimanated the truck for many years and did it in the back of a station wagon... the biggest problem was that it caused children
to commune with nature... you get closer to the wild side.
Getting back to basics is a necessity in such a limited space... I can't imagine going on a trip for more than two or three days... we are often traveling for 3 to 6 months.
Good luck... let us know how it goes... how you spouse adapts to it.
Sleepy