With 12 months to plan, this should be the easiest question to answer. Why not for the next twelve months check on what you need and use, how much you use and eat, what the prices are, and how often you like to eat out, or cook for yourself. It will give you a basic figure, which you would then have to increase for a road trip. Start a journal or log, and keep track.
It will not only be good preparation for such a trip, but a great lesson in life.
Lifey