It's easier in some parts of the country to go no-res. North Dakota for instance. I'm at a COE park in Nebraska right now and there are over 100 sites open. The camp host has the day off. Pull in and pick a site.
This trip we had reservations (or were going to a first-come place) for the next stop and I was looking for the place after that. I need to be in the Kansas City area around Oct 1, so I have the next 2 weeks booked.
I would not recommend winging it in the northeast in Just and August. Too many campers, not enough parks.