We have made six trips west since 2005 and have never made a reservation more than 10 days in advance, and that one was for Trailer Village at the Grand Canyon. We never reserve for enroute stops, just call ahead in the early afternoon each day to get a site for the night. We are retired with an empty nest and our travel plans are fluid, to say the least.
Except for one trip in September, we always head west during the months of May and June to avoid the Summer crowds when families with school age kids are traveling. It also helps that we don't need hookups every night.