...I realize the east coast is chock a block full of tourists in summer, but Sept can be such hit and miss for hot, clear, rain or cold, that's certainly not when I would plan to go to the east coast. Well certainly not the later part of September.
I usually try to do my long tours in June and early July before the roads get busy with RVs and the campgrounds start getting full. My Maritimes trip, I never made reservations for campsites, but I did for hostels. Later I suspect the campgrounds would be busier. My last full weekend on that trip was about a week after Canada weekend, and the campground I was in was at about 95 percent full.
But earlier in June, yes it was buggy and more likely to rain, but roads were less busy and the campgrounds emptier. There were several times when I was the only one on a weekday in a tent site.