I would NOT keep all your drains open. You are asking for problems especially with your black tank and drain if you do. Are you heading further "south". If so not to worry as a low over night usually will not cause problems. Put some water in the fresh water tank. Larger volumes of water take longer to freeze. DO disconnect your water line.
2017 Voltage 3970, Epic, Full Paint
2015 Ford F350 DRW CC 4X4
2016 Kawasaki Teryx 4
4 grandkids, 1 Yellow Lab, 1 Kubota Tractor