What you have is a converter that charges the batteries and supplies 12 volts when on shore power. Is there a easy way to install a invertor ........nope.
You'll have to run heavy wires (depending on the size of the unit) from the batteries. Then the unit needs a internal transfer switch to switch from shore, internal ac output. Then you have to splice it in the middle of the romex cable feeding the outlets.
Not sure what you want to power but you could purchase a small one that plugs into the power port (cigarette lighter socket)