Gasbuddy.com has a color-coded map of average gas prices, county by county throughout the country. You can also find it via RTA's fuel cost calculator on the right side of this page.
You'll probably see the highest prices of your trip in Chicago and around Yellowstone. Try to get your fuel either in Indiana or wait until you are in Wisconsin for your trip around Chicago. Yellowstone is so big that its almost hard to avoid filling up there. Prices in Gardiner were about 10 cents less than in the park last year, but stations in Cooke City were actually 25 cents more than in the park. Prices are a bit less in the Tetons, but you don't get back to "normal" prices until you reach Jackson.