Chances are that someone would have added any public charging sites to Plugshare.
There is a charging station at the ferry landing that you could use before/after the ferry ride to Ocracoke if you felt uncomfortable making it the 180 miles round trip, which if you've ever driven Rt 12, will be quite slow and flat (you should be able to easily exceed the rated range of your car).
Also, what is your situation on the island? Are you renting a house? Could you ask the hosts if they have an accessible outlet you could use? If you will be there the whole weekend, you will be able to recoup a fair amount of charge from 120V (figure about 75-100 miles per day).