I have occasion to get on the roof quite regularly. From cleaning slideout awnings of dirt or snow and ice to cleaning the roof, from servicing roof vents to checking sealant condition, from air conditioner maintenance to tying up a failed main Carefree

awning; I sure wouldn't want to haul around a tall enough ladder to accomplish it. Because of its length, I'd have to store it on the roof. HA!

No one would have to hold onto your ladder for safety if it's mounted to the coach.
I'd shop around for the best deal on a full length ladder. Depending on your engine access door and such, if you can go with one that double-folds at the bottom like ours does (the one Ed refers to) you'll be happy. Our old coach had a non-folding ladder down the passenger side rear, and a short-tongued boat trailer's winch handle got into it on a sharp turn once.
As Neil says, though, make absolutely certain it is installed by someone that understands the blocking required behind the rear cap, and proper roof sealant application where the top rails mount. Even then, you have to check escutcheon screws regularly to make sure they stay snug and corrosion-free. A loose screw can allow fiberglass-cracking movement while your weight is on it, and a screw that's too tight can spider crack as well. Any of the service outfits in Bend will have the know-how.
Incidentally, as to your original question, once every couple of years I've cleaned the roof well, and either orbiter-applied Maguiar's wax as per Monaco's recommendation, or wiped on a coat of Aerospace 303 Protectant, both of which retard UV oxidation, albeit to a limited degree. If I did it annually, it would be easier to clean each time; the black mildew-like stains are a lot of work to remove by year 2. The cleaning is the time and energy consuming hard part, and the protectant application is relatively easy. Eventually I'll probably follow Ed's protocall, but so far our roof, in the weather 365/24/7, is doing pretty good after 6 years.
Joel