Assume you have the water production in back of van.
Why not use a hose pipe to say front wall install outside Tap at front , but get a plumber to make it a push fit john guest type thing so you can connect up and shut the front door, assuming your main issue is hose through door?
Also maybe a booster pump in van to speed production up.
As your moving house in future I would not bother with the unit option as that can go tits up at a moments notice
Mr B