[S]

I have a   Pressure washer, but it not one that sucks up its own water.  I have two jobs lined up that need its own water.  Is their any way of being able to use an external pump and a water tank, to have the same affect has using a tap? ( not sure how much water pressure I would need.
The only pump I have at the moment is a 100psi Shurflow.

[BDCS]

If you have a tank in the van then just the valve or fitting at the base of the tank. This will give you a head of water if the pressure washer is on the ground outside the van

[S]

Gravity flow?

[BDCS]

Yes

[S]

But don't pressure washers that work by tape, need a certain amount  of water going through them, meaning pressure to operate properly. More than  a gravity feed?

[BDCS]

I'd be surprised if it won't work like this but it won't take long to proove either way - I have a couple of small machines that work this way.

[S]

Cheers, I will give it a try tomorrow

[S]

I just put a 25litre container next to it, has a feed. The water as gone in a few short minutes. Looks like gravity feed works.
Now I just need to make sure I put enough water in the van.