I can't confirm the Xantrex, but Magnum inverters have a 120 feed from the breaker panel into the inverter, and output(s) to various loads such as outlets. When there is 120 available from the breaker panel, it goes directly to the inverter outputs. When the 120 from the breaker panel goes away, such as when you are traveling, the inverter automatically outputs 120 from the inverter which is being powered by the batteries. It is an invisible and automatic function which takes care of itself. There is no reason to make it complicated. Let the inverter do what it is designed to do.
The battery charger in the inverter works in a similar fashion. When there is power available from the panel, the charger works to charge the batteries. When the 120 goes away, the batteries work to run the inverter. It's all automatic. Depending on the manufacturer, the battery charger may or may not charge the chassis batteries.
