The problem with plugging in the 30 amp cord:
Battery supplying power to inverter,
inverter supplying power to converter,
converter supplying power to battery.
One big loop. None are 100% efficient, so each time the voltage is stepped up or down there are losses. Then you have a lot of small things drawing power that you don't even think about, lights in microwave controller, everything that has a remote and if you forget and leave the fridge on automatic.
one fully charged deep cycle battery will not make 3 pots of coffee using a Mr. Coffee. This was direct hookups with nothing else to draw power.