Winterizing: insulating the grey/black tanks

We try to stay in central Illinois until around Christmas day unless there are huge snow storms. What I have discovered is what has already been suggested, keep some RV antifreeze in those closed tanks. We found that when the outside temp got below 25 degrees our black gate valve froze up. I run my furnace but I also use 3 ceramic heaters, two inside the coach and one in the basement.

Generally speaking when the temps stay in the 30 degree range during the day and 20 degree range at night we were filling a 30 pound propane tank every 4 days.