Very Low Fuel Avg

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HI.
I purchased Prostar 2009 Cummins ISX few months ago, it has 578,000 miles. and it was giving me very low fuel avg. it was around 3.75MPG. So i took to different shops to get it diagnosed. Everybody had different opinion about it. Since i purchased from a dealer, it had some warranty, And we found out that DPF filter was cracked so it was being replaced. and then engine was making some noise, when we open the head, it had some broken metal parts in it. Thank God it was covered under NTP warranty, so they replaced all the parts in it and the total invoice was for $19,000. Which NTP paid. and then i have changed fuel injectors on it and temperature censer etc.
After that it still gives me almost same Fuel avg. I have taken to few different mechanics, and no one could find anything wrong with it. NO air leak, no fuel leak, no oil leak. nothing
So Please give me Some suggestion, what should i do now. I am getting around 4 MPG. with no idling. i just can not afford to loose anymore money. and 3 different Drivers have driven the truck.