Don't think you have to stay on the interstate because of Hwy 89 south being closed. The detour is very well signed, it's an excellent newly paved road and runs approximately parellel to the original Hwy 89. No big hills.
Fuel: Lake Powell Convenience/Shell - 901 N. US Hwy 89 (no road closure at this point - it's much further south), 928-645-5998. It was $3.82 yesterday. Behind it is a Pacific Pride Truck Stop.
Maverik 701 Tunnel Rd. at Hwy 89 - just north of the above Shell. 928-645-2696; $3.79 yesterday
The whole route is a beautiful drive into Flagstaff. Go for it!