The no reservation thing can be rough. I however put my family through that type of adventure at least once a year. It is awesome to watch your family members face as everyone discovers America together. This is how I do it.
1. I will pick a area that we want to explore. For example last year was the Oregon Coast.
2. I will then look up the area on http://www.roadsideamerica.com/ and other detailed websites. I will also write local governments for their tourist information.
3. I will have purchased maps and possibly guidebooks for the area. Maps are important as they will provide clues for camping.
4. We take our cellphone and text CHACHA. They are a mobil search engine that can answer questions that you might need on the road.
5. Be flexible. Discoveries will be made both good and bad that will take up extra time. (Last year we didn't make it as far down the Coast as we would have liked do to unforseen limitations in travel.)
6. Don't be afraid to keep the legs small on the trip. I usually shoot for around 200 miles a trek with stops along the way. That means you don't have that rushed feel and are not spending your vacation in a car.
7. I bring my wife in on the planning that way she can get excited about the trip and sites that we can see. You could do something similar with your sons. They might find an antique train station or something like it that gets them excited.
I hope that helps you.