Now that you DO have it clean, spend the $10 and get a gallon of rubber roof conditioner and apply it to the roof. It will help seal that "white" stuff up more, and put some 'stuff'back into the "rubber" to help protect it. Think of it as vitamin oils for the rubber roof. The difference after it dries is like night and day. Also, makes the roof nice and smooth, so water, dirt rolls off much easier too.
Funny, so many will put so much attention into tires and waxing the sides of their campers and such, but do so little to help protect the most important part of their camper ... the roof, other than checking cracks in the caulking, they do nothing to the roof. Don't make sense to me. But, to each his own.