I copied DutchMan and picked up a 65 gallon sprayer tank from Tractor Supply and installed in the front of the bed, behind the cab window. Plumbed it into a marine grade impeller pump with it's own pressure switch, and used Bargman connectors on the power cord. I can plug the pump into the truck's 5th connector, or plug the trailer 7-way into the pump, running it from trailer batteries.
Most of the time, I run a hose from the pump to the city inlet on my trailer, otherwise I just use it to refill the trailer tank as needed.
Thread about my tank:
Aux water