Carrying water up high is a bad idea as it completely changes the handling of your unit, center of gravity, etc. On the roof would be the worst place, over the cab not a lot better.
Best suggestions are to use less, take an inflatable bladder that you can fill there using your truck bed (camper unloaded) or if you need more the get some tanks for beside the truck camper in the truck bed. Just remember that the more you carry the more weight you are carrying and adjust your other items to keep your rig within the weight limits.