make another outlet on the bottom of the tank and attach a length of hose from here to a pump and switch connected to a deep cycle leisure battery. From pump, attach a length of hose to a DI vessel filled with resin and from resin, a length of hose to a hose reel.
When the pump is switched on and hose control valve openend, it will start drawing water from the tank and pumping it through the resin and up to the brush head.
When pump is off, the water will stay in your tank to be used by your truckmount.