As some may know, I've always used Meguiars Boat/RV wax on the trailer. But our summers are not as fierce as yours ... and our summer camping is almost exclusively in Canada and Colorado.
Recently, Mequires has come out with a Marine Flagship
wax (Product number 6316) which supposedly has better UV protection.
I'm probably going to switch to the Marine Flagship wax once I empty all the half-full bottles of Mequiars Boat/RV wax.