No mate a sponge is the best way but you will need a brush to do the roof, be very careful with the plastic windows
as they scratch with the slightest rubbing and a brush will destroy them.
You will also need a specialist cleaner to remove any black stains that might be on the bodywork usually found below
windows or anything that has a rubber seal, also be careful around fridge vents as these are only designed to keep out
water running down the van ( don't hit them straight on ).
Fenwicks motorhome cleaner used concentrated on a sponge, I found this to be the best for removing the black marks or
Autoglyms super resin polish was another good was of removing the more stubborn marks.