I don't know about hauling 80K up a grade, but I know that hauling 20K up a grade does not require a single downshift if the driver has at least 400 HP and 1,450 torque. If the driver keeps the boost up, only traffic or curves will dictate a downshift.
I can't recall ever going below 8th gear using a 10 speed on any grade in any situation on an interstate highway.
http://onezman.tripod.com/yourrvhaulercom