i have a wired inside-outside temp gauge in the water heater-pump compartment, in the center water tank compartment and in the aft water service compartment. i found that all the compartments stay above freezing down to about 20 deg with the propane furnace set to 70 deg.
when plugged in to shore power, i found that the electric water heater element keeps that compartment toasty. i use a 100w drop cord light in the other 2 compartments.
i got my temp gauges from harbor freight for about $5 each. wireless temp gauges are available.