The compressor you refer to in the generator compartment is only a small auxilliary one, used to help keep the air leveling system functional if you auto-leveled your coach and have been parked for awhile; it kicks in to maintain level if the main air tanks get below a useful pressure, which commonly happens due to leakage over time. The main compressor is on the engine and pressurizes the tanks with dried air when the engine runs.
Unless something has changed since we bought our coach, the chuck connector you see in the generator compartment is from your tanks, not the auxilliary compressor, and as Richard and Leah have indicated, you will need to run your engine, probably at fast idle, in order to get satisfactory use from that port.
-Joel