@Doug: so no luck with the 12v direct to car battery method. Checked optical board, reseated wires going into the jumper, no luck.
But then I decided to pull the AC cable and try it on an extension cord, and it immediately switched over to AC.
Since the extension is plugged to the RV (fed from main panel), I'll assume this is a fridge outlet issue, and not AC ripple? But I was measuring good AC voltage going to the control board when it was plugged into the fridge outlet, so I'm a bit stumped.
Trouble now is that they buried the fridge outlet box so high up, it's not accessible without removing the cooling unit. Seems easier to trace this fridge supply wire back to the panel, and just install some new romex and outlet box.
Thanks again for your help!