I have a 2021 nissan titan pro4x and my mpg is 11mpg in city and 15 hwy. I use to get 16.5 city and 21.6 to 22.3 mpg

Nissan says what I'm getting is what I should get, and my infotainment center kept going out, they replaced it . I also have a wining sound in transmission in low gears

They keep giving me lip service and won't fix it or replace the truck

I have called bbb and in process of dealing with them ....what could be the problem with the gas mileage??? I used a $40 scan tool and it says the I have bad O2 sensors and bad . The computer won't give any codes for any of it?????? It started about 2 years ago and have taken to dealership about 12 times for same reason.it has 44k miles now