Duramax towing “engine reduce power”

[Willyracer]

I was towing from Cincinnati to BG. When I hit the hills of Ky my
truck shut down with the reduce power limp mode.
Had the truck towed home. I have replaced the fuel pressure
Relief valve. Also installing a Air Dog fuel lift pump and replace the 6”
Rubber fuel supply hose at the motor.
Just trying to cover all the bases
Has anyone else had this problem?
Truck runs good with no problems without trailer .

Thanks for any suggestions
David