REMOVING BROWN SPOTS FROM ROOF VENTS

We have a Coachmen Aurora Class A Motor Home and the vent caps get a brown, spotty build up on them during the summer. The unit sits out in the weather and I'm sure this is caused by the morning dew along with dirt. When I clean the roof every year, these spots also appear on the rubber roof, but are considerably easier to remove. My question is; does anyone know of a product we can coat these vent caps with the make them easier to clean? Right now I use a product call Korkay and it does a great job cleaning the awnings, rubber roofs and caulking. It does get the brown spots off of the vents, but it takes a Scotch brite and a lot of elbow grease.
Thanks for any suggestions