We just got back from a trip from Mass. to California for the Dam Gathering.
Here's a link to our photo album if you want:
http://www.jonweinberg.com/Calif2013 (requires Flash)
I roughly figured out where I'd be each night based on what I wanted to see along the way and the mileage I wanted to drive per day, and allowed a few extra days to spend more time at places that became more interesting, or to go on side trips based on info from locals where we were.
Both for planning (via their website) and on the trip itself I used an app called "We Camp Here" that was invaluable. I know that if the campground I wanted to stay at was full I could find another nearby to go to.
We mostly stayed in state and national park campgrounds. It helped that we went on the off season before school summer vacation. We might not have been so lucky during July and August. The only reservation we made was for a campground outside New Orleans as we were there on a weekend during the French Quarter Festival. Even so they were not full.
Hope this helps...