i recently pulled my grandparents' 30 foot 5th wheel (converted to gooseneck). i was pulling up some hills around 55mph and was running in tow/haul in my 06 cummins. it was never wanting to shift out of overdrive. is this normal? or do you automatically need to keep it in "o/d off"? my dad has always had a powerstroke and i've pulled more with it than my cummins and it downshifts alot, but that v-engine should just based on torque/power curves. i know that cummins makes so much of it's power down low rpms, but does this sound ok?