There is plenty of space behind the hitch for a water tank, just needs to be under about 14 inches tall. Look at all the V-notch tail-gates that are used, the king pin clears them. Then there is plenty of space in front of the hitch for an aux. fuel tank and a decent fresh water tank as well, it least in a long bed. I've also seen some owners use a flexible bladder tank that they lay on the truck roof, fill it from a faucet and then gravity feed into the fiver tank (not sure I'd put that much weight on the roof). As indicated, the fresh water tank will likely be bigger than you are accustomed to.
