I had a factory-mounted tap on the drawbar, but found it awkward to use, so I moved it to the cross-piece between the two gas bottles. The water line is plumbed in after the electric pump, but to use the tap when the camper is closed and the water pump turned off, I installed a switch wired in parallel with the original (internal) switch so that the pump could be turned on and the tap used.
The switch is a water-resistant one from a boating supply place, and it is mounted inside an old 35mm film case. It's visible behind the tap.