I generally don't do a lot of planning. I know the general direction I want to go. If I am on an Adventure Cycling route, I will use one of their maps. (
www.adventurecycling.org). If I am using maps, I stick to the old way: hard copy maps. I have found that only major routes (i.e., those with a lot of traffic) are on the state highway maps. I think you need at least 1:250,000 for a map. I have found that each state has a Gazeteer put out by Delorme. These maps are the right size, but it takes a book to cover the whole state. I literally rip out the pages that I might need and put them in my pannier. The Gazeteer shows all the back roads. It also shows commercial camp grounds and they have addresses/phone numbers so I can call if I need to. It also shows all the small towns, so I can generally rely on a gas station or convenience store being there.