When storing in sub freezing temps you need to EITHER drain everything (which is not always easy) OR put special RV antifreeze in all the lines, pump, etc.
Rather than filling the whole heater tank with anti freeze, many units have a bypass valve to keep the heater from filling up. You have to drain the water heater, then put the bypass valve in winter position, then fill the rest of the freshwater system with antifreeze.
In the spring, to de-winterize, you return the bypass valve to summer and flush the whole system with fresh water.
2008 Itasca Meridian 37H & 2015 Flagstaff T12RBST
2011 & 2012 Len & Pat's "One lap of America"
27K miles & 41 states in 13 months - Woo Woo
Yellowstone Lake 6-1-2012