We're planning to do the same thing, but in early 2014. We're going
What I've done is determine the end point and major places we want to see on the way.
I have adjusted the route from time to time. I've also added sites I like to see, when I see my proposed route goes near something we'd like to see.
I look at the area around a place I plan to visit/see, such as The Alamo in San Antonio to see of there are other places we'd like to visit.
For us, time isn't a factor, since I recently retired. For me, money will be the only limiting factor, that and how much time we want to stay away from our 2 year old grand daughter.