2006 peterbilt 379 with acert motor having power issues

Fuel mileage is around 3.4 but that's hauling 118,000 lbs total weight of cargo and truck and trailers.
The issue I'm having is that I am fairly new to trucking and this is a lease truck, kind of like a rental I guess. I'm in the oil field in North Dakota and then truck has begun lacking power and surging under load and lower RPms is the worst.
It has an 18 speed transmission and was seemingly a great running truck before this issue popped up.
Some helpful facts:
Fuel filter gauge inside truck is around 3.5 at idle and around 5 (approaching redline) at 1300 rpm under load (up hill loaded).
The check engine light just came one a a week or so ago. Sometimes it is on solid, and other times it starts flashing.
this truck sits at idle or close to it a lot. When we load from pot it's at or around 1000 rom and when we unload its the same. And that's 70 percent of the time it's running. Also at night it's running At idle due to the cold temps here.

Air filters have not been replaced since I've been in the truck (about two months). But I doubt they were replaced with previous guys either.
Truck has 626,000 miles.

It just feels like its got a break dragging or something like that because of loss of power. And routes it used to get easily up to 70 it now struggles to do fifty five. It just feels to me like I'm pushing it hard to get it to do what it did easily before check engine light. Though I've seen most trucks up here have that light on and most say its just emissions crap so they don't worry about it.

And truck mileage may be low because of all the idle time, but I checked it while running fairly long runs so idle time would have been less at this time, I put like 600 miles on it in two days which is a decent amount for the work I do.
I am the driver for the truck but the owner is a guy out of state who got to rent truck and made me a decent deal if I would run it like my own. This truck is my baby and I want to get it running right.

Please include steps of how to do the things that need done, except fuel filters and air filter as I think I can do those.
Do I have an egr and if so and I need to replace it, I'm pretty mechanical but would just need to know where it is. Most of mechanical experience is working on older Chevy gas trucks so I'm fairly new to diesel but a quick learner.

There are very few qualified mechanics in this area especially for diagnostics, so I'm hoping the guys on here can help. Thanks guys.

 

Will there be a pause between flashes to indicate the next digit in the code? also, is it likely that the air filter or fuel filters could be causing the problem? how hard a job is changing this harness out? we don't have a shop here so doing an overhead (what is this) might prove difficult in a dirt lot. but since I don't know what an over head is, then maybe it's easier then it sounds. also, what should that fuel filter gauge on dash read under different rpms?

 

Thank you for all the replies so far guys.
Big express- I'm not sure what my boost should be so I'm not sure if its a little low. But at about 1300 rpm empty I am at 30 or so, and going up a hill empty but at 1500+ rpm I'm at almost 40 boost.
Though also my fuel filter gauge is at near 5 (almost red) when I'm at 1500 rpm going up hill as well.
So questions
1) does that seem normal for boost.
2) does that seem normal for fuel filter gauge
3) Is this gauge a good way to tell when it's time to change filters.


I pulled the code using the flash code technique (thanks again) and its a code 95
I looked up the code on the sites code link post and it shows the following

RESPONDING
95 385-01 2948-01 LOW INTAKE VALVE ACTUATION SYSTEM OIL

PRESSURE
95 385-03 2948-03 INTAKE VALVE ACTUATION OIL PRESSURE

VOLTAGE HIGH
95 385-04 2948-04 INTAKE VALVE ACTUATION OIL PRESSURE

 

Does anyone know the cost of such repairs? Shop rate here is about 120 an hour or so.

And we are particularly busy at the moment but there looks to be a slowdown coming end of next week. And the shop i just checked with said they can't see me until next Wednesday. So, what will likely happen if I had to keep working until then? I'm not a fan of that idea but the company we lease onto has been known many times to fire leasers over their equipment breaking down. Apparently they think if your equipment breaks down, then your truck may just be more trouble then you're worth, since they could have filled that spot with a leaser whose equipment wouldn't break down. Not exactly genius level logic but they don't know jack about trucking.