I had a long discussion on LR4x4 about a water tank some while ago. I now have four 20 litre army black plastic jerry cans in the 90, next to the bulkhead. There is a flexible hose and pump which can be rigged for filling or pumping out. This setup minimises sloshing, algae build-up and if I get contaminated water it won't affect a whole tank full.
On the pump outlet there is a normal tap with garden hose push fitting and a purifier filter for drinking water. I have fairly long hose (stored wrapped around the spare tyre under a cover) which fits onto the normal tap to refill engine coolant, feed a shower attachment, pump the jerry cans out, or by reversing the pipe connections it can be used to fill the tank from a campsite tap, or to pump in water from a stream etc (hazards or using natural water sources noted).
This setup has been well tried in Ireland last year.
I am in the process of piping one of the jerry cans into a heat exchanger under the bonnet to give us 20l of warm/hot water after a run. This is still in development. I have managed to heat a tank of water fropm the engine with a trial rig. I don't yet know if I can insulate the jerry can well enough to keep the water warm for long, a couple of days and still have a warm shower would be good.