Mate of mine had his set up in a picasso many years ago
250 tank (one of those black water tank you have in your loft, just had the outlet/inlet sealed up) with a board on top with the hose reel mounted on it for ease of reeling in
Pump on a pump board, wires long enough to run it off the car battery
Simple but effective set up actually, and you could clean more than you think
Although by the sounds of it, you have a van mount system already...money troubles or not, I'd be using that considering it'll be the source of income to get you out of tye money problems...?