Cm 871 MPG low

[Svtrucking]

2010 KW T660 full aero
10sp 3.55 rear 22.5 tires
Cummins CM 871 455st

I’ve had this truck just about 5 Years. Since I’ve got the truck I have yet to see my MPG over 6.0 on regular. My average has always been about 5.5mpg typically drive at around 68mph running produce CA to Fl 43k in the box. I’ve done the whole egr tune up cleaning after watching the Rawze videos and also have replaced the imap sensor, differential pressure sensor. I also clean the back pressure sensor all of this has not increased MPG. I recently had head gasket go out an headcrack at about 800k miles and did complete engine overhaul. All the work was done at Cummins new head, cam rods, front bell housing, turbo, egr, egr cooler, oil cooler all the major components that go along with overhaul were changed. The DPF am DOC new as well with new doser. It’s been a year since overhaul and can’t seem to get better MPG. The water temp goes up to 210-215 on hard climbs and fan stays running. Oil temp stays above 200+. Not sure what seems to be the issue. Could it be something simple as driving? Or some other sensor or this truck is just not spec’ed for great fuel mileage. Any help would be much appreciated.