In reference to the tank heaters and what Trekwood shows one shows thermostat and the 12x18 doesn’t...
However... this appears to be same 12x18 heater pad nad it is controlled via a thermostat...Ma6nenthey make a tank heater without a thermostat but to me that would stupid...
https://www.recpro.com/rv-tank-heater-pad-12-x-18-12v/