That’s exactly what I did. Bought a 1964 Shasta Astrodome in good shape for $400, currently gutting it for an upgrade/restore. Solar, composting toilet, updated insulation, modern appliances. It’s enough space for one person, two dogs, two cats, a tortoise, and some chickens outside. Most of what’s going into it is second hand materials from places like the Habitat restore and rv scrapyards. There’s also a lot of folks who buy brand new rvs and immediately strip the built in systems like AC that are inadequate for modern giant rvs but perfect for something this small and then sell them unused for cheap. I’ll probably be $5-$8k total by the end. The real cost is the land ($75-$100k for ~.75 acre in the mountains) and the well. Lots aren’t selling as fast as they used to and I’ve noticed more price drops over the last year.

You’d pay a lot more than 25k for a 750 sq. ft. RV. Quick search suggests RVs typically range from 150-500 sq. ft, so this is 50% bigger than the largest model that range covers.