Nothing wrong with a metal roof, but keep an eye on the sems and of course watch for any sign of leaks on the interior.
There are brush on roof coatings - those intended for mobile homes will work fine. There are also products marketed expressly for RVs but I don't know if they are any better than the regular mobile home stuff. I doubt it - probably just more expensive. Most such coatings are EDPM rubber, essentially the same stuff used in manyRv roofs.
Some brands are Kool-Seal, Dewitts Cool-Top, Dyco RV & Mobile Home Roof Coat, Liquid Roof, and Liquid Rubber