I use my pressure washer for business and have had occasion to need water in places that didn't have access to water, what I did was get a 55 gallon drum, put a regular hose fitting on the bottom and would fill it up with water and drive to the location to clean such as housing development entrances and such that wanted pressure washing. As long as the tank was above the level of the washer the gravity feed from the tank was enough to supply water to the washer. My washer is a gas, 2.5 GPM@2600 psi and I never had issues with it.
Rick and Mimi
1989 Class C Fleetwood Tioga, 26 ft on an E350 Frame
Toad: 2000 Dodge Dakota 4x4.