12 v heater and thermostat for RV bays

I am looking for a way to keep my underneath storage bays from freezing while on the (cold) road or stopped. I have found 12 v heaters, and I presume that 12 v lights will work as well, but am having a heck of a time finding thermostats that will turn a system on around 35 degrees F...any and all ideas welcome...thanks.