question about battery

You household electric 120vac runs a converter (battery charger) to charge the battery. The battery is the reserve power to run 12vdc intermittent items such as your slides - power awning- power jacks and small 12vdc drain items such as lights. The a/c and water heater (if heating on electricity) requires you to be plugged in on 120vac. The battery disconnect is used to turn off the battery to everything in the rv during storage. If the rv sits for a period of time with the batteries not disconnected and not plug into 120vac the rv still has a small drain on the battery and will run it down over a period of time. The converter is not designed to run the 12 vdc power alone it needs the battery reserve so the larger 12v drain items is not pulling power solely of the converter.