Decreased power towing

[Lawrence]

Hello all,
I have a 2004 6.0. Had been running well until recently. Now if any type of a hill, I am lucky to reach the top at 45 mph while towing. The truck shifts down and pours out black smoke during that time. The turbo is reading at 28. It has all new fuel filters.
There is no missing or surging. Motor is running smoothly, just not much power.
I had it in the shop and they reported a number of error messages came up on the computer. They reset the computer and said they could find nothing else wrong.
Whatever they did, made no difference, so now, $200 later, back to square one.
Appreciate any thoughts.
Thanks