That outlet will not work without shore power, either for the 120 VAC outlets nor the USB ports, which also derive their power from the outlet itself when on shore power. There is absolutely no connection to the
battery whatsoever. Unlike the stand-alone 12 VDC "cigarette lighter" style outlet in the rear overhead cabinet, which is wired directly to a 12 VDC source, the USB ports in those combination outlets get their power from the outlet itself. No shore power-no USB power.
If you need a full-time USB, you may want to check into buying/installing a standard 12 VDC stand-alone USB, but I will also add, be sure to check the USB's output rating. Most of them may not be powerful enough to charge a laptop for example. Shop carefully. And BTW, those USB ports built into the 120 VAC outlets are generally also not capable of charging a laptop. You can charge your cell phone, but I wouldn't count on it to power larger electronic devices. Just sayin'.