Norcold "C" code

My 2011 Norcold fridge gave me a "C" code last weekend. I plug in to AC at home, let the fridge get to temp and then run on propane while traveling. Fridge fired up on propane as expected. When I arrived at the campground, noticed the "C" code. Found that indicates low voltage. Plugged into AC at the campground and fridge worked fine. When leaving, unplugged from AC and "C" code returned. Plugged brake harness into truck and fridge fired up and "C" code was gone.

So, is this a battery problem or convertor problem? My Shumaker charger says the battery is at 12.4 volts.