In my experience, 12v fridge operation creates far more problems than it solves.
I never use it.
If you don't have shore power, whether driving or camped, run propane.
My 120AC unit on my 10 year-old Norcold went out a year ago, and I ran happily on gas for that year, full-timing.
IMO, 3-way fridges should be 2-way. I act as if mine was.
I notice on the Norcold in my new camper, there is no "auto" setting.
I suspect that is because 12v operation will leave you dead in a heartbeat, and you won't know it until it's too late.
Joe