I went to a dealer to compare different sizes.
"normal" size has cargo floor over 11' long an 5-10" between the walls.
The height inside is 5-4", so not much for standing up.
Still that is more space than most campers have ;).
I think I will wait for higher roof model, who adds 12", when extended version is 3' longer - nice for the space inside, but harder to park and worse for fuel economy.
Not too many of them in this country, so having my 20k budget it will take some good luck to find what I am looking for.
Ambulances I found have lower roofs and converting would take much more, so I will stick to searching for passenger version with roof AC and extra heaters.
They come with 200 amp alternators, so running the big fridge on electric only should not be a problem.
Have seen lot of "conversions" in my search and propane cylinder under the bed, or under the sink doesn't strike me like something I would consider.