Please help! ISX15 poor fuel mileage

[Nihalu]

Hello Folks
After checking several YouTube videos on Rawze's Channel and inspecting the truck accordingly I ended up in buying a 2016 Peterbilt 389 with a Cummins ISX 15 engine in it ESN 79854924. Its a 550 HP/2050 TORQUE WITH 3.73 GEARS ON 11R22.5.ENGINE HAS BEEN OVERHAULED AT KENWORTH in my area 138K KMS ago with new rebuild kit and new cyl head. Current odo is 908000 km.Everything is just fine with it except fuel mileage. Just to mention it used to run in mountains with 140k lbs gross. Now I run vans/reefers with 80k gross. Max I got out of it is 5.8 MPG. All stock after treatment.Recently changed IMAP, DELTA P AND other recommended sensors. Dpf doc all cleaned but no mileage improvement. Please help me if you have any suggestions. Call/text 7788358132.
Any help from legends around here will be really appreciated.

Thank you so much