Good morning. I will say that I am happy with my truck. There may be differences between mine and yours. Mine has 4.10 gear ratio, free flow NO cat 4" exhaust, and a tow tune. I also have almost 170,000 miles now with no major issues.
I do tow in the mountains but not the Rockies, yet. Throughout the year we travel PA from Poconos to Lancaster and in between. We also are in and out of the Smoky mountains in TN. Oh and Adirondacks. This year we will be going to Niagara Falls. It pulls fine on the grades. What are you using to monitor your engine vitals such as oil and coolant temps? These two Temps are the most important on longevity of a 6.0 diesel. If you have an android smart phone you can get Forscan app and a Bluetooth OBDII adapter for less than $30 and be able to monitor the vitals and check/clear codes. Please ask away if you have questions. We are towing about the same weight except my truck maybe heavier @ about 9850lbs loaded with fuel.