That is a good idea Gordon! I don't know which tank has what, but assuming each tank draws relatively the same amperage, that would determine which switch serviced two tanks and which only serviced one. The next problem . . does one feed the aft galley tank and the other go forward to the black and bathroom grey tank? If I only want to heat the black tank and used that logic, I would end up leaving the heat on an empty tank (the aft galley tank) and nothing on the black tank.
Perhaps someone reading this post actually worked at the factory at or around the time these units were made, and can give me some insights. Meanwhile, I leave both waste tank switches OFF and wait (and hope) until such time as something freezes. (So far so good - the full black tank drained freely today, and it's -8 C or about 18 F.) Now that is has turned cold and is likely to stay that way, I guess I will find out one way or another if I need to heat everything. Then I will just turn both switches on and operate with a level in the grey tanks at all times.
My neighbor lives full time in a trailer without the 'arctic package' and, by virtue of skirting and maintaining heat in the trailer at all times, seems not to have any trouble.