If you are going to purchase hot box from Taybur they also have pressure washers for sale and can modify for your needs.
A Thing to bear in mind - if you fix to the floor of your van - what will you do during this weather , recently read that your pipes / tank were frozen.
You will need a Generator - bolt this to the floor of your van and unload pw every night, unless you have a lock up which will keep the frost away.