We have a built in 2-tank DI system in our coach. We use it for washing the coach, and I also use it, or our large DI system at our store to fill our fresh water tank. I usually mix in about 10% regular tap water for our tank as pure DI water is aggressive to metals. You can drink DI water, but it lacks taste and is essentially like drinking distilled water. A more typical, set up is to have a water softener for the coach (it removes the de-solved minerals in the water & replaces them with sodium chloride) and then a RO system for drinking water. To wash the coach with RO water would require a separate fairly large water tank probably at least 10 gallons. A DI system will allow you to run the water through the exchange tanks to a pressure washer without having a separate storage tank. The amount of DI water you can produce is dependent on the size of you system, and the quality of the water that is going in. I can make about 400 gallons with my 2-tank system, and it cost about $80.00 for new resin beads.