Some people with RV's buy a fitting for using compressed air to blow out their water lines, Others like me use RV plumbing antifreeze. You could buy the adapter and rig it up to use compressed air to blow water out. When I flushed my rad I found out the total coolant amounts and then dumped straight antifreeze into the rad and added some water to get the proper ratio. For me it was not necessary to blow or drain the water in the heater. I never did use distilled water, Our water is pretty good here, Better be they just built a billion dollar filtration plant.