Yes, I have learned grey water will freeze. Only had it happen once (black also froze). This was in the Class C. Slapped a water bed heater
on the bottom of the tanks (I could use one pad for both tanks) and built a temporary box out of rigid foam insulation (that one was the pink foam board). Valves were wrapped with pipe heat tape. Didn't freeze after that. In current location (NM) I have the grey bypassed and the black stays open all the time (I have a residential toilet). My supply and drain lines are heat taped and insulated. When I build the new tanks, they will have bypass valves on both (only 1" insulation as well, no heat pad). I fulltime and when in cold areas, am on full hookups. This is my last winter. Come the end of summer, I'm out of this state and heading for warmer climes.