Low Mileage krnworth T680 Cummins isx 15

[GuruT680]

My truck is giving me low mileage from the last couple of weeks. used to get around 7 mpg on empty. but now geetimg around 6 mpg hardly. Even though its empty trailer but it seems like truck feel heavy while drivimg. full loaded trailer get worse aroumd 5.2 mpg used to be around 6.2 mpg. This all thing happen after service done by kenworth shop. they said my truck was showing an actuator code so they installed new one. after that i didn’t get the same mileage i used to get. any suggestions please