It seems all of the manufacturers recommend keeping the roof membrane clean using mild soap and a soft brush, but no coatings are recommended. About 10 years ago, I had my rubber roof coated with a non-petroleum based rubber coating made for that purpose. The coach is now 20 years old and the roof is in great shape and has never leaked. So, I don't know if my preventive measure was significant in prolonging longevity or I'm just plain lucky.
I'm anticipating having the roof re-coated here in the near future just as a precautionary measure.
Holiday Rambler Navigator DP, Hummer, and Honda VT1100C Shadow