Sure, you can buy a stripped motorhome for $1,000. It is not uncommon to build a from-scratch camper trailer. You can buy an older motorhome that still has its walls (not torn down like the one shown) but water damaged, and just rebuild it. I see "Free RV" in this condition all the time. I suggest that often here, as you are right, in my view, the worst starting point to build a motorhome would be from a worn-out school bus, but obviously on this forum that doesn't win you a popularity contest. ;)
The Fiberglass Shuttle Buses are a MUCH better starting point.
And frankly, anything longer than 22ft you are better off with a trailer.