There are numerous ways to store water. The lower it sits the better, especially if travelling off-road (and I mean rutted tracks that really test the vehicle, not gravel tracks you can just barely take a Toyota on). I've unfortunately seen a number of very, very clever setups with storm water pipes arranged around roof racks with filling points and taps. They're unfortunate because putting 100Kg+ over the top of the roof is quite an effective lever above the centre of gravity and if going off-road, it is quite effective at destabilising the vehicle and causing a roll-over.
You could mount an arrangement of stormwater piping UNDER the tub although that's not easy and would probably require several separate chambers with hoses joining them together.
Another alternative is a water bladder - something like
this - but as with all flexible containers, they run the risk of damage. And if your goal is to venture out where that water is a necessity, then a bladder might be considered as a nice addition to your main water tank, but personally I'd look at the Camec tank that Geoff has linked to.
I have 3 of those sort of tanks on my caravan (1x59L, 2x82L) and with stone shields fitted they're quite safe for outback travel. You MUST treat them before storing water in them because they really taste quite bad from manufacture. Thankfully treating them is easy! Here's how.
Flush the tank with water from a good source (see the next paragraph) - put about 5 litres of water in it, swish it around thoroughly and empty it. Grab a (< $2) bottle of WHITE vinegar and pour it into the tank along with about 2-3 litres of water. Swish it around, let it sit, swish it around some more, then empty it. Flush the tank with water again. Finally, empty a packet (about $1, McKenzies from Woolies/Coles) of baking soda into the tank with 5 litres of water. Swish around, let it sit, swish around again, empty. Final rinse with water and empty - it's ready.
Hot tip for water storage: always test the water coming out of your filling hose. We did. The garden hose had horrible tasting water, because the hose taints the water. We bought food grade hose from Bunnings and half an hour later went and got our money back - it was worse! We now use a canvas hose. Imparts NO taste to the water at all. There's nothing worse than getting up in the morning and making your coffee and finding that it tastes like plastic.
Oh one final thing - make sure that nobody else has access to the water filler. Set it up so that you can lock the inlet.
