Yamaha Generator question

My friends rv has an electrical management system. Even when nothing is turned on, but plugged into shore power, his management system reads between 3amps and 7 amps. Im assuming this is his charging system because nothing else is on. If that is the case, and you have not had the generator running, the batteries are not completely charged. Therefore, when you start the genny, the charging system is going online to recharge your batteries. If my friends rv is any yardstick, this is between 400, and almost 800 watts that the charger is taking. This is between a quarter, and half of what your genny is putting out. Turn off the charger, run a cord straight to the microwave, or get a bigger genny. As stated, higher altitude steals a portion of the power as well.