I've only used my fridge on 12v a few times while towing, and though it never ran the battery down, it never really kept the fridge cold either. If I'm driving many hours in hot weather, I'll let it run on
propane. Another thing you could do is keep small bottles of water in the freezer compartment, the frozen bottles should keep the fridge cold assuming you don't open it much during your drive. If/when I ever replace the fridge, I'll get a 2-way, there's really no point in the 12v part.