The GVWR of a RV trailer once gutted down to the basic frame rails/cross members/out riggers and axles will be much less than the unit as a whole. A 5000 lb GVWR trailer may weigh 1500 lbs leaving you with 3500 lb payload. Pretty light weight for a car hauler.
Trailers made back then had no fed requirement for tags or placards. Numbers were sometimes steel stamped into the frame usually in the hitch area.
The RV main frame rails will be made from light weight structural steel compared to a factory car hauler. Same with cross member material which is usually 1/8" rolled formed sheet metal on 48" centers. You will spend money and time making a car hauler from a RV frame.
