We have been in DC for the last two weeks where day temps are in the 40s and night temps occasionally dipping below freezing. The heat pump does not heat anything really, once there is a 30 degree difference between outside temps and the temp one wants inside, so we don't bother with it. We keep water in our fresh tank, never letting it get below 1/4 full, and never dump our gray and black til full. We keep the fresh water hose unhooked until we fill the tank. We also turn on our water heater for a 1/2 hour before we go to bed to ensure the water will not freeze overnight (we are on 15 amps so everything is on propane).
We really won't worry until day time temps are consistently below 35 and night time temps are below 20. Hopefully that won't happen before January 1, when we intend to head south.