First off this had/ has absolutely nothing to do with a GFI outlet. They are 120v AC, not 12v DC.
Why did you not connect the knife switch until you arrived at camp? This switch should likely only be opened when you are storing the camper for a while with no AC power to keep the battery charged. Otherwise, sitting at your house plugged in, towing, camping, etc. it should be on.
You "ensured the batteries truly had power". How? With a voltmeter? What was the voltage? If your batteries are bad and/or dead then a brief recharge from from your generator may have made them work again briefly. If you've had the battery isolating knife switch open for a while the batteries have probably not been charged in a while.