Really, if the camper is never going to move, you can use any kind of roofing material on it, same as a house. You could go with a corrugated tin roof, or a standard metal roof that might be put on someone house, or fiberglass shingles, or rubber, or actually, anything.
A poured rubber might be OK for a while, but I think exposure to the sun over time would cause it to dry out and crack. FYI, I tried using the As-Seen-On-TV rubber spray stuff and it just peeled off once it dried. The material you put that liquid rubber on must be absolutely pristine clean in order for it to stick, otherwise you've just wasted your money.
If it were me, and I were remodeling for a permanent parked camper, I'd build a pitched roof on it and then just shingle it, like a house.