I bought these 12v tank heaters and installed them back in November.
They included heating pads for both drain valves.
I have two group 29 batteries and the second one is in the bed of the truck.
They are connected in parallel and I connected the heaters to the one in the bed.
I had an issue with the water line going to the toilet when the temps
dropped into the low teens so I bought a 12v heat tape, zip tied it to
the water line and covered it with pipe insulation.
Right now its 21° outside and the heating pads are 42°.
All of them are self regulating so there's no reason to turn them off.
I do unplug the power source once the warmer weather returns.