I had a 2014 with the 3.5 ecoboost, 200K and the only problem was I had to replace the coil packs at 135K and then a couple O2 sensors at 155K, I averaged 19 mpg, towing a 7'x16' enclosed motorcycle trailer 9.5mpg, I got rid of the 2014 a year ago and bought a 2020 2.7 ecoboost, amazing power for this small turbo, 22mpg + I only towed with it two times and didn't check the mileage, I just traded the 2020 in for a 2021 5.0 cause I missed all the bells & whistles I had on the 2014, with the first tank of gas it registered 21mpg, this 2021 Lariat has a few extra's and the interior and technology is night and day compared to the 2020 XLT.
Good luck with your search, and just like others said, how the previous owner took care of the truck will make a world of difference