I never really worried about water lines freezing up with temps just below freezing as long as daytime temps were in the 40s or higher. My rig does not have a sealed under belly and of course low drain points are fully exposed as is most of our water lines. Would have to dip to around 20 degrees for our water lines to freeze up when daytime temps are notably above freezing.
Coldest temps overnight occur for only a couple of hours around dawn, we have never experienced any issues with frozen water lines or running water heater under temps you described. Our family is in the same boat as my DW and 3 DDs are also nurses, they are obligated to work daily. They dutifully wear masks and goggles throughout their entire shift, I find it difficult to just wear a mask to go shopping.
