Its far more likely that a good portion of this mileage difference is due to vehicle configuration, driving habits and/or primary regions visited (high elevation mountains versus flat plains) or a combination of those than due to the size of the trailer. The driver of the vehicle, how they drive it and where they're driving it is one of the biggest variables in any mileage comparison. I was a member at a Mustang forum for about 20yrs and from time to time the subject of gas mileage would come up. Mind you, these are mostly all the same cars with either an auto or manual tranny and the differences in mileage would at times be stunning. People would be thinking that there's something wrong with their car when in fact its all about how they're using it. I remember one members mileage was particularly bad and after a lot of investigation we found out that his wife would sit for 20+ min every day with the car idling in the elementary school parking lot waiting for their daughter to be released from school. Its for this reason that I dislike MPG comparisons, there's just too many variables.
........................................................
It would be interesting to run a MPG poll, showing tow vehicle year/model, engine/drivetrain, final axle ratio, trailer model, lift kit present or not, typical highway speed, primary region of use, etc. Not only that but show a best/typical/worst MPG. I've found that some times people like to post their "best" MPG when people need to know their typical MPG. Some people feel MPG #'s are a contest, when it should be treated as information.
Depending on how you even record your gas mileage can make a huge difference. I record every gallon of gas that goes into our vehicles. Did I run the truck without the camper goes into a 90% pulling. For this thread I will only post my mileage where 100% of the miles was with the camper hitched.
We drive 62 mph on interstates/4 lane roads, and 57 mph on two lane roads. Only in an urban area where we would impede traffic will I drive over 62 mph.
With our 2015 F150, 6.5' box, XLT, 3.5 EcoBoost, 6 speed automatic tranny, 4wd, with a CCC of 1970#'s pulling our 5.0 for around 10,000 miles of traveling, but probably only 2,000 miles when pulling 100% of the time, we got 13.3 mpg, using the paper/pencil method, not a couple of fills using the Lie-O-Meter.
In December 2019 we traded the 2015 F150 for a 2019 F150 Lariat, 6.5' box, 3.5 EcoBoost, 10 speed automatic tranny, 4wd, with a CCC of 1730#'s. We pulled the 5.0 for 35,000 miles down the road with perhaps 7-10,000 miles pulling 100% of the time, and we averaged 13.6 mpg, again, using the paper/pencil method. This truck also had the newer version of the 3.5EB along with the 10 speed tranny probably accounting for the three tenths mile per gallon improvement in gas mileage.
Currently the same 2019 F150 gets 11.3 mpg pulling our 25' Bigfoot, a full foot wider, a pull-type not a 5th wheel, and 1,500#'s heavier.
Our friends drive the same as us, but they have a 23' Airstream and pull with a 2014 Toyota Tundra with the big engine and 6.5box only get between 9-10 mpg. He will no longer figure out his gas mileage because until he trades for a new truck it will just depress him.
In other words, unless we are all identical, this thread is about many people, with different vehicles, highway speeds, weight of camper, and how you accelerate, for different results.
Food for thought,
Perry