Been there, done that - almost.
8 hour trip leg from Olympia, WA to Boise, ID (521 miles) on less than one tank. Drafting about 2-3 cars lengths behind tall semis whenever possible and safe. Otherwise driving the speed limit (70 in WA, 65 in OR). Could have probably gotten 43 mpg out of the car if it wasn't for the extra weight (wedding presents and luggage), 8 hours on the road and the boring drive wouldn't have driven me insane. Still, 41 mpg isn't bad, right?