I've just done that to my van.
I was using a Pure Freedom Trolley... but have plumbed in a 100psi pump, New Battery - and run it through 100 metres of 6mm microbore, with a control valve at the pole.
All worked great, except I was using a shed load more water than I was used to using, and no matter how much I ' tuned' the pump could not get it to flow with a low brush pressure, without the pump going in cycle mode.
So - I took the Varistream of the trolley and wire it in - Sorted!
So much easier than the trolley, faster set up time, and no running out/ filling up barrels.
For me, I just need a larger van tank now. I'm using a 250litre tank, and carrying 4 loaded barrels.
I'm looking into getting a custom 400 litre tank made up.