As said if it's a residential fridge it should run off your inventor powered by your batteries, but batteries need to be fully charged & should have a 3rd battery somewhere.
If you have the Norcold rv fridge just set it to "auto" (AU) on the control panel & will automatically switch to gas when unhooked from shore power, be sure propane bottle/bottles are opened. The rv fridge will not operate solely off batteries, but requires 12 volt from your batteries to operate the control circuit board.
Danny & Linda
Former '13 FB owner