You should be fine without reservations in September. Labour Day weekend may be busy with some campgrounds full, but otherwise not a problem. We seldom reserve, especially through the week. The holiday weekends are the only times I'm tempted to reserve, but again we tend not to do so until one or two days out. If one campground is full, we just look further down the road, and we don't leave it to late evening to find a spot; we try to settle by 3:30 to 4:00. In September the evenings will darken early. This isn't Yellowstone, Yosemite, etc.
Enjoy your visit. If you need tools, garage, support or whatever when you're in the vicinity, get in touch. (Located half way between Halifax and Chester.)
Stephen