My camper weighs the same as what your FIL is looking at and my wifes truck does not have any trouble with it. Hers does have the 3.73, but it does not have the max tow package. The steepest grade I have pulled with it is 221 up Linville Mountain and it did fine. 55 mph was not a problem at all. This is probally not as steep as what you have in the Rockies, but my gut feeling is that your FIL would be fine with the camper he is looking at.