Hi Sean,
My caravan is practically brand new compared with yours.....( being not quite 20 years old!)
I find the best thing is to use really hot water (as hot as the hand can stand) in a bucket of water with car/wax/wash shampoo added at double the suggested dose!
I apply this by long handled brush from my trusty aluminium steps, and it really does work on my old ABI
It seems to leave a good coating of wax onto the roof that seems to "seal it" from the normal dust, dirt, leaves, bird droppings and general gunge that seems to accumulate on caravans during the winter etc.,
After you have given it the treatment once ( and removed a good proportion of the gunge) it becomes a lot easier for subsequent applications... the green algae, and general gunge seems to just float off after a while and with little effort compared with the elbow grease and effort needed the first time.....
I keep my van in the garden ( under trees) and when I first got it, it had so much green slime and gunge on it that I thought I would have to drag the lawn mower up there to deal with all the growth!
I use the cheapest carwax/shampoo I can find (from Asda) and managed to buy a large 2 or 3 litre container that should just about see me (or the caravan out!) I think it was only about £2.50 or so.
I do the caravan with it before the start of the season (around Easter for me,) again in June or July, and a final going over before the winter starts, Some folks use Fenwicks which is good but fairly expensive, but I've found the Asda stuff and very hot water for the really stubborn growth and marks works wonders... once the roof is reasonably clean the water only needs to be warm not hot.
I'm not sure that the manufactures or valet firms would approve of using my technique, but I took the view that with a fairly old van it was worth taking the risk.... I've been doing it this way (3 times a year) now for the last 8 or 9 years with no problems, but as always it your call..
I find the most difficult part of the whole process is now.... avoiding falling off the steps!
Good luck, and of course,
Happy Caravanning
paws