Take the cover off on the inside and make sure that the outlet (cold) air) is not recycling with the intake (warm) air. Remember that the air being cooled comes from the inside of the RV not from the roof. In some cases the two plenums are next to each other and the metal can separate and cause internal leaks and recirculation. Use aluminum tape to repair.
Other generic tips are, find shade, and run the A/C hard at night when it's cooler out to "sub cool" the coach. This may allow it to keep up better in the daytime. But it'll get cold at night.