Any chance for Topsail without a ressie?

We are in the midst of planning a trip south in October. I know it's late to try and get sites this close on, but do the State Parks reserve 100% of their spots out or do they make sure to keep a few open for daily "walk ins". We would love to visit Topsail for a few days but we are working on a "wing it" schedule. Our plan is Orlando,(for which we DO have reservations), then somewhere on gulf-coast (Topsail?), but then might like to come back up and visit Stone Mountain, and then go over to Pigeon Forge. We have never had the opportunity of so much time to travel so we want to make the most of it, yet don't want to be strangled down by a schedule, and hope that there is at least a chance of getting into one of the many state parks without a prior reservation. Anyone else done it? Thanks for any help.