Run your heater. We were in Anza Barrego a few years back and the temps dipped in the low 20's at night. We certainly run our heater in those temps. My fresh water hose froze, but no damage.
We noticed a gentleman across from us with an older motorhome and he had the hatch to his water heater off with several folks looking at it. I walked over and noticed his water heater tank had split open. We ran our heater at night; he did not. He commented that he new something was up when he got up in the morning and the water in his coffee pot was frozen.