Roof coating

The 5 YO roof on my Coleman is starting to show black across the trusses, which is the coating wearing off. Last fall when I washed and protected the roof I tried scrubbing to see if it was on top or under and found it was under. I had forgotten about it until I was up there the other night measuring the AC unit height while the cover was off to determine which AC replacement could be used so I can park in the barn without taking the cover off and then remembered about the roof.

So what would be a good coating to use to help protect the rubber being exposed.