All gas trucks drop down to 10 mpg or worse, when even the lightest trailer is towed. This should be no surprise to anyone. The worst towing fuel economy I've ever experienced, was a bull moose, camp gear and boat on trailer running from eagle Alaska to Wasilla. 7mpg. This was a 3fe powered landcruiser. A dog sht engine.
My buddies 2019 freightliner did better than that, towing over 30,000 lbs of sht through the mountains.
Best fuel economy I've ever experienced towing, was a 16 ft twin axle trailer full of windows, lumber and stove pellets. Stayed at 55-60 mph. It was a 1992 w-350 with 6bt Cummins and a five speed: Almost 17 mpg. Empty, that ugly fkn thing would pull 21 mpg.
Uncle pulls a 40ft toy hauler with an older but pristine Volvo semi (N14) for the same reason. 12 mpg loaded, never has to drop a gear to pass/go up hills etc. A gasser or diesel pick up isn't likely to get the same MPG AND stop the thing with any margin of safety like that truck does. And given the cost of a new F350 these days - he likely saved 50k on the purchase price...
