Water tank size.... What would you do?

The under the bed “bladder” sounds like a good idea. Also, a bladder or two in the truck bed is also a great option. The under the bed bladder would be an especially “great” idea, if you camp in very cold weather. We use a 55 gallon plastic drum for extra water.....a real pain to keep water in the “liquid state” when camping in single digit F or colder temps.