Most RVs I have seen with residential fridges use a 4 battery setup. 2 batteries are the norm for units with a gas fridge. You should not need to upgrade the charger/inverter/converter. It is obviously big enough for the 120 VAC you need and should have no problem charging a couple extra batteries. Remember, when you are dry camping, there are a lot of other electricity draws. Watch TV? Satellite receiver box, furnace fan if it's cold, smoke and CO detectors, water heater control circuit, lights, maybe more stuff. 12 hours on 2 batteries sounds about right.