As a lot of our camp areas are primitive, we take our water with us, so have one like this: 5 US gal = 19 litres. Put a load of ice in with the water and it keeps it cool for a fair amount of time, even in our 35-40c summer temps.
I carry two 20 liter water canisters, but they take up space, and are quite a nuisance to take in and out. I'm planing on welding together 4 metal 20 lit. jerry cans and fitting them under the floor between the chassis rails. That is valuable unused space, it benefits the weight distribution of the bus, and i don't need the tanks in reach. Some places that i've traveled, water can be pretty hard to come by, you need as much as you can carry.
i use a 2 or 5 litre bottle of water from supermarket 17p/99p took out my westy water tank for the space, how often do you camp where there isn't a tap close?
Mine is a 15L collapsible type with a tap, from go outdoors only cost a fiver and no leak..... So far http://www.gooutdoors.co.uk/cncontinental-foldable-tank-ghirba-15-litre-p207143