Suggestion - call each park directly and ask if they have first come, first served sites. Many parks hold a few sites out of the reservation system so they can accommodate people passing through. If you can't get in the day of arrival, you may be able to dry camp somewhere then roll in first thing in the morning to claim a site. Once in, they usually let you stay. I know it's dicey and you may not get in but we haven't missed yet. Now if it is 100 degrees out and nighttime temps are in the 80's, you might want to pay more and guarantee hook ups or plan to run your generator overnight in the Walmart parking lot so you don't bake.