2016 Prostar no codes or pulling power.

I have a 2016 prostar with the n13 motor and 10 speed manual tranny. that has been down for going on 3 weeks now.
Backstory: I was hauling a load from Atlanta to Charlotte and the truck was running great. I got about 10 miles outside of my destination and all of a sudden it feels like my truck has no pulling power at all. And the rpms wont go over 1200 in 10th gear. Every little incline and my truck is dropping from about 65 to 45 or 35 mph. I didn't hear any loud bangs or anything sounding like the turbo going bad. I stopped and checked for any loose hoses or clamps and didnt find anything. Sitting still the truck reved up with no problems. Ended up making it to my destination that even running about 55 mph max speed on flat ground and any little incline dropping me down to 35 to 45 mph. Got unloaded the next morning and took the truck straight to rush in Charlotte. about a week later I get the truck back with a new fuel rail and cac tube. Luckily covered under warranty. I Drove the truck about 20 miles to my house with no problems. Went to pick up my trailer to go get my next load and on my way to the shipper with an empty trailer my truck does the same thing. roughly 20 miles or so. I go to go up a slight incline running 70 mph and the rpms wont go above 1200 and my speed starts dropping like a rock. Once I get to the top of the slight incline I am at about 45 mph and the truck wont get above 1200 rpms and 60 mph. So I limp it back to Rush in Charlotte and tell them the truck is doing the same thing as the first time. This time a get it back a week later and they changed the fuel water separator housing stating that there was air getting in it somehow causing the fuel mixture to be off. So I take the truck for a test drive with my empty trailer about 40 miles or so. The truck runs great. No problems at all. I went to pick up my load about 50 miles away and the truck is still running great. I got loaded this morning and drove the truck roughly 40 miles and the truck started doing it again. It is acting like it has no pulling power at all and the rpms wont go over 1200 in 10th gear wide open throttle.

The real kicker is that through all of this I have not had a check engine light or a mil light come on yet. I am at a complete loss as to what is causing the problem and I think the mechanics here. at rush are scratching there heads was well. They have hooked there computer up and tested the turbo and said it was good. They keep sayin that it is a fuel problem and not a turbo problem. I am pulling my hair out and dont know what to do next. Luckily I still have my warranty on this truck so the parts aren't costing me anything. but the time I am down is really starting to suck.

Lets spitball some ideas for me to check or ask the mechanics..... @Heavyd

 

If someone could tag HeavyD in this post I know he is the go to international guy.

 

Rush in Charlotte has the truck and the last thing they told me was that they pulled codes and found 157fmi2 is inactive. they have disconnected the FRP sensor and checked for damage and found none, and measured the FRP was in specified range checked low side pressure and found it greater than 30. Installed clear hose and found no aeration in low side fuel system, and that they need to continue more in-depth fuel system diagnostics. I have sent them a message asking them if they have put the truck under a load to replicate the conditions and symptoms I am describing to them and haven't heard back yet.

 

Is thee a way that I can get an navistar diagnostics program? If so which set up would you recommend?

 

They got back with me this afternoon and said the high pressure fuel pump is going out and it needs to be replaced. I hope it fixes it.