I would drain the HW tank they engage the bypass valve. You can purchase an adapter that screws into the hookup for the fresh water hose. This is the water hookup as if you were on city water. This screw in adapter has an air valve intake on the top and you hook up your air hose at the point. One could also use the winterizing hose on the intake side of the water pump and snap a dust or sawdust blower nozzle on your compressor hose. The nozzle would fit in the bypass hose you would normally use to draw RV antifreeze through the pump into the water system.