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
97 DR650, 05 525EXC, 01 XT225, 89 TW200, 09 TTR125, 07 TTR90, 07 CRF50