we bought 3 wired inside-outside temperature gauges from harbor freight for about $5 each. there are wireless temp gauges available. the wireless installation would have been easier.
we put them in the front water tank/water heater compartment, middle tank compartment and the aft water service panel compartment.
we found that the front compartment does not require supplemental heat as long as the motoraid, electric, or propane water heater is on. we use a 100w light bulb in a trouble light in the other 2 compartments to keep them above freezing.
when the propane furnace is running, it keeps all of our water compartments above freezing.