My Yaesu typically reads 12.2-12.8 when the main battery is at a "full charge." The battery won't start the truck if below 11.8, I have to flip the switch for that. It's also nearing the end of its useful life, so it tends to wear down quicker than it should. In Moab I was able to keep the fridge on all day and all night, and the truck always started the next morning... though one day I did have to flip the switch. Also, it was kind of cool at night..
Toss a set of jumper cables into that big tub o' parts you have, and quit worrying about it.
As for the fridge itself, if I kept it on 3 it pretty much froze everything. 2 seemed about right, but stuff would still freeze at night... 1.5 is a little more like it, but a lot depends on what's in there- frozen water bottles tend to require less power than warm beer cans..
82 fj40, 99 uzj100, 06 gx470
05 525EXC, 01 XT225, 07 TTR90, 07 CRF50