Duramax tows like my F-150
 

I just purchased my first diesel and took it on a road trip towing my 5,000 pound travel trailer. The truck runs awesome empty but when I'm towing I feel like it lacks power. Maybe my expectations are too high but my 2008 F-150 was able to tow up the hills at the same speed as my Duramax:td:
It tows ok at lower speeds until the trans temp hits 200, then the fan comes on and I lose a ton of power. Going up a gradual grade I can't even hold my speed (usually 70MPH) by the time I get to the top I'm struggling to go 60 (these are not big mountain passes, just gradual slopes). I can't imagine that this is normal. Is it?
I took it to a duramax mechanic and he checked fuel pressure, boost, injectors, etc... and said that nothing was wrong. I refuse to believe that a stock Duramax can't tow 5K up a gradual grade at 70MPH.
Any ideas on what could be going on?
The truck is stock except the AFE Bladerunner turbo inlet.
Thanks for your advice!

Fill out your sig with all your info, it will help us better diagnose your problem. Lack of power is either fuel or air. How is your fuel filter or air filter? Depending on what year your have you can flip the transmission cooler and get more air over it to cool it down. Think you can only do that on an LBZ though. I haul a 34 foot 5th wheel and can get on it pretty hard but I have so extra stuff installed....of course I am throwing the dreaded 0087 code so I have yet to find the problem out on mine.

The first thing I did with mine when I bought it was dump all the fluids so I could start with a baseline to go from. Might want to do that because you never know how long some of that stuff has been in there.