It is not necessary to "blow out the line" just let it drain normally. It is recommended to drain the line any time it is near freezing even if the camper is in use. The one battery will run the fridge as long as the 7 pin plug from the TV is plugged in to the RV.
A single battery will not run it long by itself. maybe a couple of hours. Also when plugged in to the TV, if the engine is not running, it may run down the TV battery if left long enough.
2015 Pinnacle 36 FBTS
Cummins Onan RV QG 5500 Generator
2015 Ford F350 Lariat FX4 CCLB DRW 6.7 PSD.
Curt Q20 Hitch. Air Lift 5000 Ultimate Air bags. Airlift Wireless One compressor.