We visited this Park in fall - the last week that the ferry over was running.  There are roughly 40 sites there and there were maybe 4 other campers there - This is in big contrast to the summer when it's packed.  I would call this a walk in site and not a hike in campground which makes is accessible to more folk... and allows one to bring in more gear than if it were a true hike in site.  Wagons are provided to haul gear if desired, water is available at the entrance to the campground.  We were in site #9 - on the water and pretty far from the dock - a lovely site in the dunes.  We spent 2 nights - allowing us to relax and also walk all the trails..... It's a small island so this is very doable.  We had been to Rock Island on several earlier visits but had never camped - glad we did - especially in the shoulder seasons - Don't think I would like it as much in summer, too crowded.... I'd head to Newport State Park first - much more solitude - and many more miles of hiking.  The boat house on Rock Island is stunning, worth the trip just to see that, and the light house also cool