I think the difference will be very noticeable, myself. If you spend most of your time in the flats the difference will certainly be less noticeable than if you do any mountain driving. Then you will really see the difference.
I pull a 3000# loaded TT with an F150 5.4 and I spend a lot of time in mountains. It’s a piece of cake to pull, I can maintain a good speed and I don’t high rev up the grades. I have no doubts at all that if I were pulling 5000 or 6000# it would be a whole different ballgame and I’m quite sure I wouldn’t like it.
I don’t know if fuel mileage would differ that much, as said, wind resistance is the culprit at speed. I get the same mpg towing as I do solo in town, but you don’t drive 65 or 70 on mountain roads so slower speed is likely helping there.
I don’t have much success with cruise. I use it when I’m on really flat ground, but when I get into any grades it kicks out. And the transmission hunting for gears is hard on it so I usually don’t even bother to use it.