First, we don't go camping. We are RVing, fulltime, so this is not 2-4 month trip, this is our way of life.
We make a reservation for our winter stop in Mesa, and then about a month before, we make sure we have reservations for Memorial, 4th of July, and Labor Day. Otherwise, we have a vague idea of where we are going and a general time frame and we start out. VERY, very seldom have we had trouble getting a spot. We've been known to change directions when we find out a friend is just a day's drive (~200 miles) away in a different state than we are - we go join them for a few days and explore that area. Then start off again and see what comes up.
Barb