Between Kingman and Flagstaff is a bunch of hills. That's going to be a handful in a slower than anticipated deuce, especially with someone not used to the peculiar skillset these trucks demand.
He should fill up in Kingman, and fill up again in Seligman to dilute any badness. He may very well notice an improvement in performance with further driving.
@Mos68x is in Seligman, with knowledge and a 939series 5ton.
Albuquerque and surrounding area is going to be another high stress, traffic and hill infested mess.