we do some touring vans and a couple of static vans on a site. We charge £20 for just a wash down on the outsides. For the small vans. If the insides need doing we charge for that accordingly. It is easy money as they are quick to clean. Don't use a pressure washer if I were you as you can easily damage the seals. If you are doing vans with the plastic windows be very careful as these will scratch really easily.
I use my vikan brush on the static vans and I leave the roof aswell as they can leak. This was a tip I picked up from the forum and when I mentioned it to the owner of the park he was fine with it.
I use fenwicks caravan cleaner and it comes up a treat. But apart from that I just use me wfp gear.