If you want small, but still full living quarters, look for a pony express unit. They built a bunch of units on Chevy 5500 chassis (and a few 6500) with an 8' box and everything one person would ever need to live in. Full bathroom, shower, fridge, stove, sink, tv, a/c, generator, slide out with sofa, and bunk over the cab. I don't think I'd like 2 people in one, but great for single.
Here's one for sale in my neck of the woods:
TRUCK FOR SALE, PONY EXPRESS TOTERHOME PX800RF. LEONARD TRUCK & TRAILER: 800-455-1001.
I don't remember the mileage, just that it was low. They want $65k, and I know they were over $100k new.
This one sold, but it has some good pics of how they are laid out:
Pony Express Toter 17,495 Miles!! For Sale