tuning question

[ptlogan77]

I am new to the forum, so please excuse my ignorance if this subject has been covered.
I have been a lifelong yellow motor guy, but last year I purchased my first Cummins since I had an NH220 42 years ago.
These red motors are quite different than a cat, and I'm trying to learn as I go.
The Truck is a 2018 Peterbilt 389 with a CM2350 X15 CPL4343 that has the EGR, DPF, SCR, and DEF system removed. I have kept the VGT turbo, and have no complaints with it.
The proper Cummins coolant bypass pipe has been installed in place of the EGR cooler, and the ECM has been tuned by a tuner that was recommended to me locally here in Washington, and is supposedly set at 565/2050.
The engine has 20,000 miles on an in frame (liners, pistons, rods, mains, oil pump, new head with Inconel valves)
The specs on the truck are: 280 wheelbase, Flat top sleeper, RTLO18918B transmission, 3.25 rears, LP24.5 tires, rolls down the road at 60mph in direct gear at 1600 rpm. I pull a van and a reefer.
I know with the long wheelbase and flat top sleeper I'm giving away some fuel, but I'm struggling to get 5.1 mpg and on my most recent trip to Boise and back (1200 miles) I averaged 5.2 running half the trip empty!
The current tune keeps my EGT's below 800 when running below 1500, with plenty of torque to run it down there, but above that on a long pull I get over 900, and I'm aware that the VGT turbo is not a fan of high temps.
In my feeble brain, it seems that the ECM must be getting more fuel at higher rpm, causing the extra exhaust temp, which seems counter-productive for my goal of better mileage and not having to lug the engine.
I'm looking for help with a better tune for my application.
I'm not looking for a generic "Hot rod tune", and am wanting to learn as I go. Any help or direction would be appreciated.