Actually, all the sizes you mentioned are used in RC Car/Truck bodies.
It depends on what you are doing... some of the extreme lightweight 1/12th scale bodies are made of .020.... it's possible a few were even made of .015 but only the "Pro's" used them becaue they tear/tore like SaranWrap, most of the lightweight 1/10th scale bodies (oval competition type) are made of .030. Most of the Truck or recreational bodies are .040. SOME can get heavier.
I'd been in the RC Car/Truck Hobby business for almost 18 years, but its been over 3 years since I got out now, so my memory may not be as good as I think....
Kevin (formerly known as "Y-City Hobby & Speedway Inc." in Zanesville, Ohio)