Depending on the construction of your RV
Fresh water with 20's at night you may not need do anything to the fresh water system. PROVIDED you have heated tanks.. You can also get tank heaters that peal and stick on the tank if you wish or in my case since the tanks are enclosed and there is room I slipped a string of incandescent C-9 (9 watt lamps) Christmas lights under the tank.. They put out about 300 watts. and a 100 watt rough service lamp in the valve bay (Caged so it can not touch anything and melt it) NOTE 15 watt lamps will melt plastic 9's do not.
But I have all enclosed plumbing..
Water heater.. Either drain and bypass it. or leave it on and hot (hot water will NOT freeze so long as the heater is heating.