Question for the guy's with duallies and big campers

Thanks for the input guy's. That was more than what I was expecting and I'm glad to know I'm on the right track. What are your speeds when driving through the mountains? My F250 is a 90 with a 5.8l and with the camper (1,500 empty weight)and a trailer pulling two atv's the best I can manage on steep Colorado mountain grades is 35-45mph. The engine in this thing is beyond tired.