No need to plan in advance. Set your starting and ending locations and map out any cities or places on the way you'd like to visit. Set off in the general direction, and buy maps at gas stations as you go. Plan your route each night inside your tent. Supplement your paper map with your phone map as needed, or if you get lost.

You can plan as detailed as you want, but if your tour is more than a couple of weeks long, I guaratee you will fall back to what I have just described eventually.