PhilH,
You sound like the type that avoids blue roads and major urban areas.
Unfortunately, you have missed the two best planners ever produced....
I am referring to to Strips and Treats and Street Alas. With either, you could lay out a route and adjust it as you learn. They would be telling you where to start looking for fuel and a place to crash for the night. They were also the best enroute navigators ever if you bought a GPS puck to plug in. Like having a big road map with a "You Are Here" arrow on it. But, I digress, they are both dead and there is no replacement as a stand alone. Don't even mention RV Trip planner. That is an expensive online service and if you don't have wire (internet access) 100% of the time it is useless and can't be a navigator at all. It can do as well as the others like Furkot, which is a free online app.
Your best solution (IMNSHO) is to get (or have a friend get) a USA map from AAA. I nice soft pencil and a big eraser. Figure out how long on the map scale is a 3~400 mile day's drive and start dreaming....
Read all the guide books you can and ask friends and relatives that have been there what they thought.
Most of all, keep dreaming.
Matt