I would go with a generator, either installed in the engine compartment on tempary strapped on the swim platform. The propulsion engine does not need to be running for either of these options.
Two batteries will not last long, run the numbers. What is the amp hour draw of the AC? You will need an inverter that can achieve this with 10% margin. Then calculate the amp hour draw on the batteries to see how many hours they will last but take into account a 10 - 20% efficiency loss through the inverter and that once the batteries discharge to a certain level the voltage will drop to low for the inverter to provide adequate power to the AC.
1997 2400 SCR
5.7 Vortec / Bravo 2