In our chunk of the world mould also seems to like to grow and parking the trailer year round under trees in our driveway doesn't help.
Every spring I clean the top (white panels only) with one of those padded teflon type pot scrubbers - works well - takes it off quite easily. Once off I then use one of the deep cleaning type polish cleaners and apply a hard carnuba car wax (again - white roof panel only). The deep cleaning polish takes out any staining of the white paint that comes from leaves - this kind of staining does not seem to be problem on the aluminum of the rest of the trailer.
Spring cleaning of the roof portion of the aluminium end caps is with a rough towel and typical car wash soap - comes off not quite so easily but not a lot of work either.
Rest of the trailer is cleaned with a soft car wash type mitt. Then all waxed - usually Rejex on front panels and Walbernize on the rest.
Should also point out that I use an old toothbrush to remove and keep mould down on the crevices of the trailer - i.e. belt line, window mouldings, around running lights etc.
Trailer is now close to 15 years old and for the most part still looks great - close to new (minor exceptions are a bit of road rash, filiform in a couple places, and some lifting of the clear coat in a couple places).
I would really caution against using the power washer - my gut reaction says that high pressure and rivets, caulked seams aren't going to go too well together.
Bambi - 2002 (The Toaster)
Pathfinder - 2009 (The Buggy)
"I'm not young enough to know everything ....."