Interesting and useful discussion. For my cross country ride in 2007, I used a combination of ACA maps and my own mapping. My ACA maps were the Northern Tier to the Great Rivers, which I followed south to the Katy Trail in Missouri and then headed west again. In the West, cyclists have far fewer road choices, so finding a route is a simpler business. East of the Mississippi, life can get pretty confusing for the pedal powered people. I was EXTREMELY grateful to have the mapping worked out by Adventure Cycling. I'm sure other good routes exist, but knowing which ones are bike friendly, where to find services, etc. is very difficult for non-natives.
Of course, one can figure out routes, but the research is time consuming. One great resource is to check if the states you are riding through have already produced cycling route maps. I've used such maps for Oregon, Arizona, and Kansas. I'm sure others must exist. Once you've narrowed down some choices, post questions in places like this forum and others. You'll get folks like Westinghouse who've ridden EVERYWHERE.
Don't dismiss the ACA maps because you think they'll be crowded. In fact, as one poster said, running into other cyclists is a pleasure and a relief at times. It's a chance to connect with a wider touring community, swap stories, and get some up-to-date route info. I looked forward to all the encounters I had--not too many, really.
Whatever method you choose, enjoy!