They are thermostatically set once the switch is turned on, to operate between 30-40 degrees, on/off respectively. There are 12v powers so they should only be used while hooked up to electric as they will drain your batteries quickly. I never used mine as I found the heat inside kept the fresh tank operational and the black tank is located inside and I did not worry about the grey tank. Also since you should be hooked up, I found using a heated water supply hose more useful than the pads.
Sometimes life gets in the way of living......