The title said how often SHOULD you... I'm fairly certain your owners manual doesn't have any requirements about cleaning or conditioning your roof. I know that if it is a Dicor roof, the manufacturer has ZERO requirements about cleaning or conditioning.
The rubber roofing material is designed in such a manner that washing isn't needed. It is designed to sit out in the sun and not leak water. It naturally chalks off the white material over time and still does not need to be cleaned.
The only portion of your roof that requires any maintenance at all is the caulked seams where the roofing material ends and the sidewalls of your camper start. That is the only thing that you need to worry about cleaning and inspecting. Water and a brush are more than enough to clean that to inspect. I usually have Eternabond tape covering that caulked seal, so I never needed to worry about any of that.
My old trailer was 17 years old when we sold it 2 years ago... never once washed or conditioned the roof. In fact, I think I only washed the walls once the entire time I owned it...
I have washed my new trailer exactly twice in a year and a half... just because its new and I feel compelled to wash it. The roof... never washed. At some point this year I plan to Eternabond the seams, so I will wash the edges in preparation for the tape.
That being said... I'm sure you can wash and condition your rubber roof as much as you can afford. Just make sure that you don't use a product that contains petroleum distillates, or anything else specifically excluded by the manufacturer.
