Royal Truck bodies is one place that makes such custom bodies for pickup chassis.
El Tec is another brand. Both are located in a suburb of LA, CA.
You might want to check with your insurance agent. Sometimes a pickup is considered 'normal' and they can write an insurance policy for it. However a custom body to them looks like a 'commercial vehicle' - even though your intention is only personal RV use.
You might want to check with the
Escapees.com Club who is made up of many full time RV'ers. They have links to insurance agents that specialize in coverage for class 6 and 7 trucks used in RV use, for a reasonable price.
Good luck on your search. There are many members of Rv.Net who have special body beds on the pickup or class 6/7 truck frames. Some of them might chime in with where they bought their truck bodies.
Fred.