Though the icemaker water solenoid works off 120 volt AC, the heat tape protecting it is 12 volt battery powered, so it works even when the rig isn’t plugged in to keep the water-laden valve from freezing. Our tape comes on automatically around 40 degrees F. Right behind the vented access door, the valve the solenoid controls is exposed to freezing. I failed to drain the “stored” coach’s icemaker valve one winter here at home, and for a while had the battery mains shut off; the next spring when hooked up and turned on, water went everywhere behind the fridge cabinet due to a cracked valve. It was relatively easy to replace, but still a lesson.
You might try using an IR thermogun like used to check highway tires, or perhaps even an IR forehead temperature sensor, to verify that heat tape is working below 40F. Ditto for your bays.
On our model warmed engine coolant is pumped through the hydronic heat unit so as to save using its diesel burner so much. I don’t think there’s a “Coach Heat” switch, as it’s apparently automatically circulated through the HydroHot exchanger when the engine runs. But as Steve notes, some models can have such a switch, convenient to the driver for taking advantage of warm engine coolant on the road when the dash heater can’t cut it, and the living area thermostats are on. There’s also commonly a preheat switch to use when breaking camp to warm the engine on very cold mornings/days. Switched on 20 minutes or so before starting up, a pump in the hydronic unit circulates warmed engine coolant back to the cold engine block, essentially Coach Heat in reverse.
Joel