The only "planning" I do for fuel, is to determine how many miles I'll be driving, than add another 25% to that number for site seeing. I divide that number by 10 (I get 12-14 mph though), and then multiply that number by the approximate price for diesel at the time. This gives me the total dollar amount I can anticipate for fuel for the entire trip. I then make sure I have funds to cover that amount. Most times, I come home with money still in my pocket. Still, it's a good average that works for me.
Meanwhile, selecting those fuel stations? No, I don't plan ahead. We stop about every 2 hours (sometimes more), somewhere for something. If we've traveled half a tank of fuel, I fill up. I try to never go below half a tank. Because we stop often, finding fuel stations is never a problem... anywhere, and my rig is almost 60 feet long, bumper to bumper.
Get the Gas Buddy app on your cell phone. It's priceless when traveling! That's all you need. No planning required. Plan your over all cost, and then go and enjoy! Drive by the way you feel. Stop often. Your on vacation. Enjoy it and don't over plan it. Planning ruins it, especially when plans get changed by different events!