The only stretch where you might feel isolated is between Flagstaff and Page as US-89 passes through the Navajo Reservation, and even there Cameron offers basic gas/food options. Note that Phoenix to Flag is all Interstate. If you will be returning to Phoenix, then I would heartily recommend that you take a different route back, just for the variety. And you can't get any different than returning via I-15/US-93 through Las Vegas.
AzBuck