I use DeLorme Street Atlas. You can set your estimated mph for different types of roads, the types of roads you want to avoid or prefer, how long you want to ride each day, how big is your gas tank, your mpg, and let it calculate your route for you. Stops for sleeping and for gas will be estimated, based on your data you've entered. You can fine-tune your route by entering vias and planned stops. It will produce a triptick (like you'd get from AAA), a route map, strip maps, and a GPS file you can download to your your Garmin. If you stay on the route, it works great! But you'll miss seeing LoVel and Bonnie-Girl in that parade you didn't know about.
Best suggestion -- allow enough time to get there. Cut the pages out of Rand McNally for the states you'll be travelling through. Get a general idea of the routes you're planning on travelling, and go!