full 120v
but 12v lights are dim
sounds like your converter is dying
might try a portable battery charger connected to the batteries
because if the 12v gets to low the fridge controls will shut down
also could be a bad battery dragging the converter voltage down
you test this by disconnecting the batteries (temporary test), just removing the main negative ground will work, if the lights get brighter and stay that way, converter could be oK, but bad battery
if they stay dim, bad converter, and you need to charge up those batteries before they die and not fully recover