We have the 12V heating pads on our grey and black tanks just in case we want to camp without hookups in cold weather. Also perhaps more importantly, 12V pads can of course be switched ON when driving in cold weather to keep the tanks from freezing while going down the road.
The 12 pads are supposed to consume around 6-8 amps, each, while they're operating. However, they cycle ON and OFF so they don't consume current continuously.
Running the pads over cold nights when camping without hookups should probably be done with only at least a 200 amp hour coach battery bank ... that gets charged back up daily.