That's right, caravans are liable to leak and they then rot inside.
I would reccomend one of the extending car shampoo brushes to clean with. You can mix the cleaning product in an oversize bucket to get the brush in.
I have used a product called 'Bobby Dazzler', (they make caravan cleaning products) in a pressure sprayer after cleaning. It helps the water run off and as you rinse off with clean water it leaves a nice shiny finish. Quick leather to finish off .....stand back and admire

You will probably need a set of steps just to do the roof.
Hope it helps
Les
Here's one I prepared earlier