Individual state maps are pretty good. Some state might even publish maps specific for bicycling. Iowa has such a great map what tells you the traffic density on the roads so you can pick nice country roads. All roads of this map are paved. You can get it at most bike stores. I guess I could send you one if your path goes through Iowa. Otherwise try to see if the state's tourist office will send you their road map and maybe inquire if their state has a specific bicycle maps.