I have a tankless in my 2017 Greyhawk and really like it. You have to know how to operate it properly. Tankless needs correct pressure and constant flow.The typical brass pressure reducers for RV's will frequently drop the pressure too low. You need an adjustable pressure regulator like a Rinaldo to maintain 40-50 psi in the rig. You don't need to add any cold water. The tempearture is set at the thermostat(105 degrees) and you just turn the hot on full and leave it that way. Nice long hot shower. I leave the grey valve open and attach a hose so the grey doesn't overflow. I have a Girard, but if you retrofit I would go with the Truma. It is able to be cleaned of scale easily and the Girard is not.