If you want to warm up the sewage to 60F, go ahead and turn the switch on! I think they're one of the dumbest things on my RV. The non-adjustable thermostat will turn the heating pads on when the temp drops to 42F and keeps them on until it senses that the tank has reached 60F.
They're 12VDC powered and draw 7 amps each, not good for boondocking! There are two pads on each tank on my 32-footer. Since we're rarely out in the cold weather and the tanks are empty when the rig is parked for the winter, I've never used them.
I don't know what the effect of 160 watts of heat over a fairly small area of the tank would be if it were empty. It wouldn't surprise me if they got hot enough to melt through. I've disconnected the wiring on mine.
Frank and Eileen
No longer RVers or FR owners