CHeck this website, and try to fill up at the lowest cost gas stations. You can input your destination, MPG and fuel tank size, and it will give you a list of the lowest price gas stations along the way.
Flagstaff is not known for low fuel prices, but Page might have some. Mount Carmel Junction is probably a good place to get diesel fuel. I would get at least 20 gallons, if only to avoid fueling up near Flagstaff on the way home.
Ruby's Inn is a great place to stay while at Bryce. Enjoy the trip. The canyon walls change to wonderful colors just before sunset, worth the wait.