Never cleaned the caravan roof, ever! Why? You can't see it and rain keeps bird poo etc off it, you don't clean underneath it so why on top of it?
Polish? I use Autoglym Caravan & Motorhome Cleaner with a wet sponge and hose it off or take it to the local car wash but I've never used polish and whenever I part-ex my caravans the dealers always comment on how well I've looked after them.
It is kept indoors though when I'm not using it.