As NRALIFR has said.
The 12v in 3-ways was designed to be used only when driving, and to "maintain" temps. And, over the years, many were poorly installed, and killed many batteries.They should be hooked up so the element only works when the vehicle is running, and now, they use modules that shut it off if voltage gets too low, usually about 13 volts. I added 12v to mine, RM2501, because I like to experiment,and I used one of the relay modules that they use in the UK and Australia, set the voltage to turn off the element at 12v. It can keep the fridge at 35f, but I have only used it 2 times, I expect it will not when it is 90-100f out. I have a small inverter, in case I have to take a long driving trip,and will use it on 120, especially in hotter weather,while driving.