Sounds like someone applied a coating of mobile home roof coating, commonly called "cool-seal." It was probably put on either because the rubber roof was damaged or there was a leak. Getting it off without destroying the original rubber roof will be next to impossible. I would just apply another coating of the same stuff. You can get it at Lowes or Home
I have had at least two slide-in campers with the roof coated with that same stuff. It just needs to be recoated every few years, three to five years if I remember correctly.
2010 Sierra 1500 Crew Cab
2009 Funfinder X 210WBS