The 80 has been sitting since Saturday with the fridge running, and I checked it tonight and it still seemed good. Voltage on the battery was 11.4V. I cranked the temperature down to 27* to get the compressor to cycle on, which it did and ran steadily and happily for 5 minutes.
So the problem definitely stems from something in the 12v socket plug setup I had previously. With 14g wire and the Anderson plugs the voltage drop seems to be minimal.