If the OP's truck has a pickup bed on it from the factory, then it is not detuned. Chassis cab trucks have to go through a different emissions certification than complete trucks, and this difference in emissions certifications is the reason why they make less hp than complete trucks. Basically, chassis cab trucks have to control NOx at full load while complete trucks do not. This allows for the complete trucks to make more power since the EGR is not used under full load like it is on chassis cabs.
OP, even big mighty diesel trucks will slow down with enough weight, drag, and grade. As stated before, I would lock out 6th when towing that heavy up grades. Like mine, your truck has two overdrive gears, 5th and 6th, and in my opinion 6th is overdrive for unloaded or small loads while 5th is overdrive for medium to heavy loads.
Now if it generally doesn't slow down up these hills and it is just starting to then that is another issue and the first place I would look is your fuel filter. It may be clogged and you are not getting enough fuel to make the power needed. If you have a way to check actual and commanded fuel rail pressure then this will let you know you are either not getting enough fuel or the truck is defueling to keep temps in check. If it is commanding more that actual than you are not getting enough fuel and likley have a clogged filter. If it is commanding less than max(26-29k psi) and actual mirrors what is commanded then the engine is defueling itself to keep temps in check. Another issue may be turbo overseed which the turbo for those years was notorious for, but it would have thrown a check engine light for that.
2014 Ram 2500 6.7L CTD
2016 BMW 2.0L diesel (work and back car)
2023 Jeep Wrangler Rubicon 3.0L Ecodiesel
Highland Ridge Silverstar 378RBS