While dirt itself may not effect the life the roof, you can't imply it helps it any either. Maybe I am wrong, but below are my theories
1) A dirty roof is more susceptible to the growth of **** like mildew.
2) Water won't run off easily like a fresh waxed car, causing puddling and the above mildew. Dirt will hold water like a sponge vs running off.
3) A dirty roof will eventually make the sides of your camper dirty when it rains and the dirty water pours over the side. I also wax the sides of the camper once or twice a year as well. Nothing like having the side of your camper smooth as a babies bottom.
I am not up there with a power washer and stiff bristle brush. A simple soft brush and roof cleaner goes a long way to keeping your camper happy imo.