The vast majority of RVs either have a Rubber roof or a Fiberglass roof. there are some with metal roofs, and even some with roofs made with composite synthetic materials.
But they all must be cleaned as recommended by the manufacturer.
Most RVs, until recent years, had Rubber roofs. The roof material used was the same as that used on mobile homes. It was designed to flex where necessary, and last 15-20 years.
What many people don't know is this material is not actually rubber, but a composite, and is designed to flake.
By this I mean that the white (if dirty, then black) residue you have running down the sides of your rubber-roofed RV, is the outer layer of the roof material flaking and washing away, thus allowing the roof to remain flexible.
Of course, those white and black streaks down the side of your RV are ugly, and they need to be cleaned off regularly. The white is the roof particulate itself, but the black is actually dirt from the roof.
To clean the roof itself, and this should be done every couple of months, you need to get the proper rubber roof cleaning chemicals from your dealer, or a camper supply outlet store.
These cleaners are designed to clean the roof, while not damaging the roof material itself.
Do not use any type of sealant on a rubber roof. These will cause damage to the roof material itself, and should be avoided.
Your Rubber roof is designed to flake over time, and this must be allowed to occur naturally, regardless of how much of a nuisance this is, when cleaning your RV.
Most of the newer RVs have fiberglass roofs, and are much easier to keep clean than the Rubber Roofs.
Check your owners manual for your specific RV manufacturers requirements, but typically, you can use standard automotive car wash chemicals on your fiberglass roof with no problems.
And, of course, there are several brands of fiberglass roof cleaners available at your local camping supply outlet stores.
Pressure Washing the Roof:
Pressure washers are often used on either type of roof, with no detrimental affects to the roof if done properly.
Of course, be sure that the person operating the pressure washer understands that care must be taken when using these high pressure devices around the ACs and other accessories mounted on the roof.
Care must be taken not to crack the seals around vents, ACs, antennas, etc. These devices on the roof are generally sealed to prevent leaks. And, the sealing products, are often of soft, flexible materials such as silicone, and can be damaged easily, or even blown away, by direct water spray from a high pressure sprayer.
And, actually, a rubber roof will clean just as easily with a soapy brush as with a pressure washer, with less opportunity for generating roof leaks around accessory items mounted on the roof.