We average three months out West every other year without reservations. Last summer we went to S Lake Tahoe on Labor Day without a reservation and found a site at our first choice BUT it was on Monday not Friday. Being self contained helps and thinking outside the box on places to overnight...one of my favorite is small airports, helps.
We usually only have a general idea of where we want to go. Last summer it was WY and CA ending up at the balloon fest where we did have reservations. If we need FHU we will call midday and make a reservation since we have a good idea of how far it's to our next stop. We had hoped to get to the S Rim on Thursday but we stayed at the N Rim a day longer because of weather so all of the CGs that take reservations were full BUT our favorite type... USFS that do not take reservations a couple miles south of the Rim had plenty of sites. By Saturday it was fairly full.