Would it not be cheaper and more simple to buy a water carrier and submersible pump?
I see that the Explorer Group, who fit relatively tiny tanks, are now fitting pressure systems to their new vans so that a water carrier can be parked outside on site, a la caravans.
Mind you, hark who's talking - my van has a 130 litre tank, as standard, and I do tend to nearly fill it before leaving home - it does last a long time.
I did do a weight check and the loaded van, full of water, was well within the limits.
Paul