2006 CM 870 Cummins ISX fuel mileage problem... stumped

I recently changed companies and am now running for a dry van fleet on a dedicated contract that has me pulling 8-10,000 pound loads one way and going back empty. In the last month, my fuel mileage has slowly dropped to 5.5 mpg. The engine is in an '06 379 Pete, with an 18 speed and 3.70 rears, rolling on 11R 22.5 rubber (XZE3 up front, XDN2 out back, I know they're not great). I typically drive no faster than 60 mph. A couple of months ago the shop I usually deal with said two injectors were on their way out (cylinder power imbalance) so I started saving for injectors. Now it's in the shop to have the injectors done, but the mechanic called me and said "You sure you want me to do this? There's nothing wrong with them." So I said no, not yet, but please find my fuel mileage problem. He called back about an hour ago to tell me that everything (charge air cooler, intake boots, turbo, injectors, EGR valve) tested like brand new and he has no explanation. Anybody got ideas?

 

Been awhile since I posted this, so here's the update: an old grey bearded mechanic at that shop found a slightly loose coolant temperature sensor wire to the ECM, so he fixed that. The explanation is that the ECM was reading the coolant temperature as being cold, and so was fueling the heck out of the engine to warm it up. He also told me that regardless of what anyone else has told me, that engine wants to be run hard. So, on his advice, I drive it like I stole it, 65 mph everywhere, and guess what? My 30 day average mpg is now 7.22.