You probably have a defective volt read out on your usb outlet. Just another thought, do you know for sure that the usb outlet is wired to the chassis batteries, or could they be wired to the house????
I would start with the voltmeter checking at the usb outlet for, input voltage. Are you only getting the high voltage reading on the usb readout? If you do have an actual voltage going above 15vdc, as said above, it will start effecting all kinds of equipment. You definitely have something going on, but need to understand what your actual vdc readings are to share more ideas. Don't want to send you on a wild goose chase. Check voltage running and not running, plus at the dash and at the batteries.