I don't know why you would want any 12v mode for your fridge, since that eats so much battery.
Our 1981 camper we got two years ago, had one of those old 3 way Dometics that only worked in AC. The LP had a pilot that worked but it would not go over to main burner from pilot. Result was, we tossed it and went with a new, small, 120v only fridge.
Recently we got a used Dometic 7401L that is also a 3- way but we only want the 2- way. The advantage is the LP on this one does not need a pilot. Plus the freezer and shelving is way better than the old ones.
Perhaps you can get the new fridge working by doing the trouble shooting in its instruction manual?
If not, and you really want the 12v element to work in your old fridge, I suppose you could try to swap them. It would be way better if you could get the new fridge working, and swap the whole fridge.
