It depends on how big the tank will be. A small tank, like those in the solar showers is fine. But if you're talking about a large tank, then I don't think you would want that much weight that high on the vehicle. Most builds have the water as low as possible because you want your center of gravity to be nice and low. Water is probably the heaviest thing you will be carrying in your car. I recommend keeping it low. When I was considering moving my spare to a swing-away bumper, I have a metal-fabricating friend who was going to build me a compartment where the factory spare goes, and install a plastic water tank. The metal shielding would just be to protect the plastic while on the trail. My plans have since changed, so I never did it.