Mill lamp flickers on and off

I have a Cummings ISX Peterbilt 2012, with an APU unit. Recently my mill lamp keeps coming on and off. I just changed both fuel filters at the Cummings shop. The shop said there was nothing coming up on the codes therefore they couldn’t find anything wrong. I lose power going up hills.

The truck feels like its starving for fuel and skips. Also when driving, it kicks in and out at no particular speed. Just wondering if it’s my secondary fuel pump, because I have already had my primary fuel pump replaced.

Any suggestions on what anyone thinks its maybe.