Another option in addition to the exposed light bulb, is keep water running at a trickle. If your sewer hose is connected, leave the gray tank valve open and the kitchen faucet running at a trickle. Movng water doesn't freeze. Well it will, but it has to be really cold. I would also wrap the outside pipe with some insulation.
I have spent a number of winters in the freeze zone, and use a heated supply hose, this really helps, I made my own, but Pirit sells a good one.
For really cold I have even wrapped the sewer hose, plus insulation, but we were seeing -20 deg.