03 4x4 Eddie bauer w 3.73 HD tow - w a 4k trailer and load, 8x12 enclosed I saw minimal loss on a 150mi trip, mixed city/hwy, I averaged 13.2 vs the high 15s I usually see. I was hauling furniture and the route was very close to my daily work commute, but w 2 runs.
The one time I towed a boat for the in laws from indy to Dale Hollow, 21' Caravelle w a road cover and twin axle trailer it was more like 9.5-10mpg. I probably could have made it there on fumes but obviously a bad idea when towing and already knew of a sizable station to stop at about 75mi from my destination. That was around 4400lbs if I recall w trailer weight and gear in the boat. The back roads once getting off the hwy really killed it. Many curves and speed changes. The trip was made at typical speeds, 65-70mph hwy and probably averaged 40ish on surface roads.