I stayed overnight with a friend at his private campground a couple months ago. You buy a lot that's basically a campsite and you do virtually whatever you want with it.
The lot I stayed at had a $10,000 shed they converted into a nice little cabin for a few more thousand. They laid linoleum flooring, built a loft with 2 twin beds and some storage, put an A/C unit in the window, and outfitted the joint with electricity, some cooking equipment, shelving, a kitchen table, and a queen bed. So it easily sleeps 4 people and could be a fully functional living quarters for a minimalist.
It was pretty badass. I was inspired.