I am not suggesting but STATING that something is wrong with your truck!! That is why they traded it in! My 2019 6.2L and 10 Spd averages 13-14 mpgs pulling my boat to lake house at 75 mph. My older 2017 5.3L with 8 spd did the same. So you getting 8 down to 6 is a-little suspicious. Now are there trucks out there that just get terrible mileage because of this and that?!? Yes, but very far and few.
You couldn’t pay me in the past year to buy a used truck. With the crazy inflation and lifter issues that isn’t being addressed the correct way. (GM should Replace all of them right away when there is an issue, instead of one side at a time from what I have read and heard about here on forum).
I would go back to the “stealership” you bought it at. Ask for all of its prior service records and have them pull it up to see if it’s been rebuilt. If it had, then ask to have them look it over, then I would push to have them work with you on extended warranty if you are keeping it.
Is it just me? But they “ stealership” should inform customers on used trucks what is wrong with them! Especially the lifter issues. Isn’t anything $5k or above in damage supposed to be? With inflation redoing a motor or half a motor has got to be close to pushing $5k.
Hopefully you get some answers. The good thing is that with high prices on used trucks. You might be able to get almost all your money back out of your truck to trade in for a HD. But they are over charging for used HDs right now. Cousin just bought a 2020 Denali and it was only $2500 less than a new one but the new one didn’t have heated steering wheel, cooled/heated front seats, and heated back seats. Chip shortage. He did get a lifetime powertrain warranty with it though so that a plus.
Good luck.