I met a guy whose advice was based on more experience than i shall ever have. He had a great beard as well. Really full and white. Anyway, he swore the best way to carry water was to use a series of individual jerry cans, mounted in a rack. the rational being
you can clean them easy
you know how much you have
you are fairly leak proof for desert traval
they can be carried if you ditch the truck ... happy thought!
they can be filled from wells and stopcocks by hand.
you can replace them if damaged or soiled.
you can manage how much water you carry and how long you carry it.
etc etc.
i still use a internal tank. when i live in the deserts its a pain in the ***, to fill my tank i have to move my truck. Pretty much the internal single tank is a pain in the *** anywhere apart from petrol stations. I would be very interested to hear thoughts on the subject. Perhaps both a rack of jerry cans and an internal tank...?