My 2004 3500 Duramax loses power while pulling a 35 foot 5th wheel and speed dro

I had a similar problem with my 2005 GMC 3500. I replaced the fuel filter about 4 times in 1 month before coming to the conclusion it wasn't that and it wasn't the fuel injectors that the mechanics were telling was the problem. Everytime I changed the fuel filter, I also pumped up the pressure (bleeding air out of the fuel filter) as is normal when changing the fuel filter. The last time I lost power, I pulled over to the side of the road, turned the engine off, then simply pumped up the fuel filter pressure.... Presto... full power once again.... Over the last 3 years, I will still lose power once in awhile, but only when I really push the truck. Hope this helps.