I don't soft wash really but have done some. I just set up a pump fitted to a 25litre barrel and fed wfp but ....
If u serious and looking to do it a lot why not set up a tank in the van, just use 150 litres but if looking to do loads set up a 650 litre. Use it the same way as wfp set up but with no filters. Run it out the tank, thorough the pump and into hose reel that feeds the wfp, using fan jets in the brush.
Use a proper pump with vital seels tho .... Well I think an so everyone else says but I just used an old shurflo pump which done the job