If your inverter is a Xantrex of around 2006 vintage -- your control panel likely looks like the one pictured here:
https://www.westmarine.com/buy/xantr...6?recordNum=19
Just press the INVERT button to switch the function to inverting the 12 volt storage battery voltage to power the 120 volt circuits connected to this inverter, such as a residential refrigerator. There is a transfer switch in the inverter that will pass the generator power to these loads if your generator is running.
Chuck