Several years ago, I purchased a high pressure water blaster. One of the reasons was that water restrictions banned the use of hoses for car washing but a water blaster could still be used in conjunction with a bucket. In my case, I fitted a hose connection to the bottom of a 20L bucket, which could then connect to the water blaster. I could then easily clean the car from that one bucket of water.
There are a few caveats with water blasters. The sealing of an RV is designed for the pressure from a tropical rain storm or a household water supply. No matter how good the sealing is on your van, a close spray from a water blaster runs the risk of forcing the sealant out, thus making a nice little entry hole for the rain. Even underneath the van, using a water blaster is going to cause some problems. Apart from getting in through the floor, high pressure water hitting and penetrating a wheel bearing will almost always cause problems.
So I recommend you keep your distance, and use the water blaster for washing off soapy water rather than the actual cleaning.
For the full story, read Caravan World #463, on sale February 18.