On long trips we usually plan to stop between 4 & 5:00 PM. At about 3:00 PM using either Streets & Trips software or a Good Sam's camp directory we’ll call ahead to campground in our anticipated area. Only once in about 700 days of camping have we called and all said that they were full but then one of them called back and advised that they had a cancelation. Other than on holidays or in high tourist area you’ll probably not have a problem if you’re stopping by 3:00 PM as a lot of unreserved campers call it a day after 4:00 PM.
Dave