Go to the F150forum.com and many get poor economy as the 3.5L Ecoboost. Some say they get 28 mpg. So most likely if you drive like a grandma and stay out of the turbo, you'll get mileage similar to a 2.7L V6 and when in the turbo, mileage similar to a 5.0L.
I have more power on both ends, torque and HP with my 5.0L and get pretty good mileage. I get about 18.5 mixed and 21.5 Expressway. Towing I got around 10.5 mpg with no wind and about 9.5 mpg with high winds.
Many complain about the 2.7L engine shutting off at stops and restarting when the brake pedal is released. Annoys the **** out of many to the point they jumpered wires to keep the engine running permanently. There is a switch to keep it running but it has to be pressed everytime you start the truck. The big gripe is that the AC goes off as well in 100 degree heat and why many want the engine to stay on.
BUT next year the 3.5L Ecoboost will have the engine shutoff feature as well.
I like the 5.0L the best. Instant go and plenty enough torque to tow my trailer. I like the characteristics better than the Ecoboosts.
In 2015 dealer was stocking mostly 2.7L and 3.5L Ecoboosts. 2016 the dealer is stocking mostly 5.0L. They usually stock what sells the best or what customers want.
2019 Ford F150 XLT Sport, CC, 4WD, 145" WB, 3.5L Ecoboost, 10 speed, 3.55 9.75" Locking Axle, Max Tow, 1831# Payload, 10700# Tow Rating, pulling a
2020 Rockwood Premier 2716g, with a 14' box.
Previous 2012 Jayco Jay Flight 26BH.