There is an automatic transfer switch that activates when AC Power is sensed at the front connector. When AC is sensed at the front connector, (from either shore power or a Generator), this transfer switch disconnects the rear receptacle AC power to make sure power to the internal circuits, only comes from a single source.
You can attach your generator to either connector, or your shore power cable to either connector. The circuits in the Airstream cannot tell the difference between Shore Power and Generator Power, they are both 110Volts AC 60HZ. Either works, you just don not want both at the same time.