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Photos(Google)and elevation data(www.rwgps.com)make route planning nearly anywhere easy. In as much depth as you like. A resource most overlook is Warmshowers hosts. If you're a member, you can call any near the route for advice about a specific area from someone with first hand experience. Link to state tourist bureaus for free state maps and literature.

When using the above resources, I get just enough info to verify the reasonabliness of the route, but not so much that I've done a virtual tour before ever rolling out. I don't want to know in advance what's over the next hill or around the next curve. Once I'm satisfied with the route, I transfer it to my gps as a track file.

You can, of course, just wing it with a paper map. The old fashion way. Plan as you go. You'll get there. I've done several of those.