There’s a lot that can be done with a decent frame and skin on an RV.
I’ve gutted and rebuilt two Airstreams in the last 20 years, wish they had been mine.
Using expanding foams in the walls and floor, rerouting the hot water lines and venting, adding window covers (seals), and more the second one done could be heated in 30f temperatures with the hot water heater and fridge alone. Below that one needed to turn the heater on, high wind days would effect it but it remained livable.
Labor intensive? Probably a couple hundred hours research and planning. Then there’s the actual work, but that can be compared to buying a similar unit if it existed.
The first unit ended up in Texas, post hurricane. Replacing a lost home, its still there.
The second is used as a seasonal home by an author friend in the mountains of Colorado. Its literally stored inside a building made of straw bales, and the only reason the owner doesn’t stay there year round is it takes a snowmobile to get there for 4.5 months of the year.