I am looking at purchasing a 2003 to 2007 5.9 cummins dually. I see a lot of hot shotters using these, the guy that delivered my fifth wheel, who does it for a living was driving one. Without any mods will a Gen 3 dually 3500 handle a 36ft fifth wheel (11,035lbs empty, 13,000lbs loaded), pulling mountains in Colorado and Wyoming? I am also interested in hearing about what mileage you are getting and what flat highway speeds and mountain pass speeds you are getting? Also any upgrades that are a must have? I really am not into modding the truck, with programmers and such, I just want something reliable that will handle the load. Also looking for input on automatic vs. standard for towing with these rigs. I am not that familiar with the 48re, but if the advice is to overhaul it with some mega trans kit for 3k I am not into that either. I have been down the beefing up, modding road with my last truck and have broke that addiction.