I don't have a motorhome, but I do cover my 28' trailer, relatively easily, without getting on the roof. The short version is this. We lay the cover out flat, then fold the sides inward to make it approx. 5-6'wide. Then starting at the rear of the cover, roll it onto a 2" x 6' piece of PVC pipe to act as a spool. Then slide a 1.5" x 10' piece of PVC through the 2" pipe. Slip on a 1.5", 90 degree elbow on each end of the 10' pipe, then insert another 1.5" x 10' PVC pipe into each elbow to create a giant "U". This allows us to lift the rolled cover up onto the front of the trailer, tie the end to the trailer tongue, then with one of us on each side, walk the "U" down the length of the trailer unspooling the cover down the center of the roof. Once unspooled, the folded sides easily unfold and fall down each side.
Probably a poor description and there are a few other small pieces to the PVC contraption, but I won't bore you with more details, unless interested. PVC is cheap and it works for us.