An example of our experience over the 15 stops we have made since leaving FL the beginning of April and arriving in Ukiah, CA today. We stayed a week at two of those stops and five days at another. Reservations were made for those three early on. Of the rest, some we made reservations for the day before the stop and the others no reservation. Of those we made reservations the day before, two didn't have room for us but we were able to find other parks that did, one we got their last site and one had no FHU but had an overflow spot with 50 amps and water. On one of the occasions we just showed up (Ft. Stockton, TX), we arrived at 1:30 and they had no sites big enough for us that were not already reserved so we had to backtrack 10 miles to another that had only two sites we could fit in.
Long story short, there are a lot more rigs on the move this year than we remember seeing in the past 10 years and if you want a site at a desired park, you need to make reservations.