RV Antifreeze or Windshield washer fluid depending on the temp and length of storage
Used to deal with this when powerwashing unheated hog buildings in the winter, made up a suction hose with garden hose fitting and 3' length of hose, disconnect water supply hose, hook up short hose and stick into the container of washer fluid rated for -20F or whatever was available. Disconnect pressure hose from the pump and just let it **** most of the container thru, you can tell when the washer fluid is coming out of the pump. We were going to be running it again the next day and generally inside the building was in the upper 20's or 30's so it didn't get that cold. Wouldn't get too worried about hoses for most overnights but we would make sure to unhook the gun, keep it vertical and keep the trigger depressed with not safety inspector compliant zip tie so that the valve wouldn't freeze
If it was being put away for a longer period, we'd blow out the pump with a compressed air adapter we made, then **** down RV antifreeze.