Poor gas mileage

[Learjet 1]

I have a 2015 Tahoe that has gone from getting 20 MPG to 16 to 17 MPG. I travel from Wichita Kansas to Cedar Rapids Iowa at the same speed (70 MPH), same route, and stop at the same gas stations each trip. I made 3 round trips and got between 20 and 21 MPG on the Highway. Took the car in for recall on air bags programing. And the next three trips I got between 15.7 and 18. Before transmission would only downshift maybe 6 on 7 times on the whole trip up or back. Last trip back I keep track of it and the transmission down shifted 37 times. Took it to Dealer and they said nothing wrong with transmission. I told them I didn’t say there was, I had told them I think the engine is not putting out the same HP/torque as it was before. I think there is an injector, coil or something they did in the programing that is causing the engine not to put out the same power as before. All I am getting from dealer now is lip service. Called GM and said I wanted to talk to a tech Rep. One hour on the phone with kid who didn’t have a clue where to send me or what to do and at the end all they said was take it to the dealer. What kind of mileage is everyone else getting. I have a 98 SRT Jeep that gets better mileage that this Tahoe is now.