Herk is right in his first statement. But to expand on that, when you first start drawing current through the 50' Home Depot cord, it is cool and resistance is low. Thus you will see very little voltage drop across that cord and the AC will be within the 20 amp draw that it needs to run at the house and RV breakers. Over the next ten minutes though, the cord heats up, and this provides more resistance. With the added resistance, there is a larger voltage drop across the cord. As it was stated earlier, the AC wants the same amount of watts. So therefore, since voltage at your house outlet is still 115-120, it's only calling for 18 amps plus maybe an extra amp or so to keep the cord hot while at the RV breaker, you just hit 20.5 amps.
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