you can use a pressure from a tank in the van a lot of pressure washers draw there own water so if you where to put a hose from this type of pressure washer into the tank it will draw the water.
what make/model is the washer
if it is a pull start you can pull it over without switching it on so it wont start and put your finger over the water inlet to see if it sucks if so it will draw water from a tank
what i have done is adapted a small water tank its about 50 ltrs with a stop cock and hoselock fitting at the top and a hoselock fitting at the bottom this way you can connect the customers tap to the top and the pressure washer to the bottom fill the tank before you start and you are always 50ltr's in front of yourself so if there pressure isn't good enough you are feeding from the tank and every time you stop washing the tank is filling again until it is full
better than using pure water