It's not too expensive to have electric pad heaters glued onto the bottoms of your grey and black tanks to keep them from freezing - although you're probably storing your RV with them empty, anyway. However, the electric pad heaters also do make it possible to camp in freezing temperatures when the black and grey tanks have liquid in them. FWIW, I'm not a fan of RV's that require the propane furnace being used to keep the grey and black tanks from freezing. The heating pads approach is much more practical in that it doesn't require running of the propane furnace. The heating pads even come in 12 volt models for drycamping in freezing temperatures.
Probably/hopefully all of your freshwater lines and tank are in the interior of the RV. By using an electric heater inside with all doors open that might have fresh water lines anywhere in behind the areas that they open up to, you should be good to go if you keep the heater turned up high enough. For a large RV, it may take a couple of heaters near each end of the RV.
If your RV has any part of it's freshwater systems outside the RV's interior, then I guess you have no choice but to winterize the freshwater system.