Pick up an in-line filter with a rechargeable canister. I've got mine mounted to the side of my house, but it sounds like you might have to take yours with you, so a little stand and a shorty hose would work.
I've just got a single stage, but here's the style I'm talking about
:
Deluxe Clean Water Filter Kit, 2 step water filter system, water filter for car washing, inline hose filter
You can get them at about any home improvement store. To recharge the medium, I just store the filter in saltwater (one of those plastic spaghetti storage containers is perfect) and then just hook it up for car washing.
Wish I could find where I got mine from, but I think it was only about $50.
My 'car wash' hose also runs through my water softener, so that helps as well.