I can't really speak for all, as I've mostly had small trucks for many years.
Several weeks ago dragged a trailer of about 2500 lbs., gross weight, a little more than 500 miles. My 4WD 4-cylinder Toyata Tacoma's usual highway milage dropped from about 22-24 mpg. A few weeks before that we used my buddies 4WD 6-cylinder truck to drag about a 3500-4000 gross weight trailer on that same route. His normal 18-21 highway mileage dropped to about 8-9 gallons on the same 500 mile trip.
My guess is that if you had a bigger truck than the ones I mentioned above, especially with a diesel, I'm not sure you would see such a dramatic drop.
Ken
