Get a 2.7 eco with a 10 speed. Its a beast as well. I've driven both 3.5 and 2.7s. There is no need for you to get anything more powerful than a 2.7 for single sled hauler. The 2.7 gets incredible hwy mpg as well when not towing. The used 2.7 with a 6 speed is a good option too. My dad has that.(2015) The other guys are right about the ecoboost engines being better at elevation. Even a 2.7 ecoboost vs a GM 6.2 has more power at elevation. I'm not saying its better, just stating facts.
I have a 19 f150 XLT sport 4x4 2.7 10 speed. 36 gallon tank, and a Rear electronic locker. ( all factory options) I just bought a 7.5x20x5ft V Neo NASX enclosed trailer. Have only towed it home from the dealer about 100 miles, but it pulled it great up steep hills @ 75mph no problem. Good luck in your decision.