If you want to stay IN popular parks, especially during heavy travel months, then you will have to have reservations to ensure you get to stay there. If you do not mind staying outside the parks and driving into them each day, then there are usually private parks within a reasonable drive that you can likely, but not guaranteed, get a site without a reservation. May not be a place you would prefer, but at least a place to stay.
You will just have to plan to be very flexible. Keep an RV park guide book handy and/or, if cell service is available, check rvparkreviews.com to find parks local to that area and call ahead to save driving all around looking for an opening. And in worse case, expect to maybe spend an occasional night in a parking lot somewhere.
