Should be a easy tow. I pull a 7500lb travel trailer and while noticeable it's not to bad. I have the 3.5l eco boost. Wind is the big issue and you won't have that issue with a flatbed.
I am getting 10-11mpg with my 28ft trailer as long as I'm not climbing mountain passes. I think the frontal area and shape has more to do with mpg than weight in most cases.
At 28ft and 7500lbs this is as big as I would go with a f150. If you want a easy tow more like a pop up, stick to 19ft or less. The truck has plenty of power and braking, but things get crazy when windy out.