I have not noticed white particles on the driveway, just black gritty looking stuff. I wash my roof everytime I wash the trailer, after every use, but I had not "cleaned" it for a year. When I wash it, I use Gel Gloss RV Wash in hot water, same as I use on the sides. I wash it from a step ladder with a long handled soft brush. When I "clean" it, I get on the roof and use a Rubber Roof Cleaner, and a nylon bristle scrub brush following the directions on the container.
Sounds like several folks are using the liquid Spic & Span general purpose cleaner rather than a specific Rubber Roof cleaner. From what I'm getting it seems like the frequency is more important than the product.
Just wondering if anyone else considers "cleaning" the roof different than "washing" it. This is only my 2nd RV, so I'm still learning. I appreciate the sage advice!