Some older data that doesn’t directly apply, but I pulled a 9k TT with a 2002 8.1gasser for several years. It never bypassed a gas station, I got ~7mpg towing. Pulled fine on flats, and OK on small hills; got stuck slowing down on a pass for a trucker and could never climb back above ~15mph while the engine screamed all the way up the mountain out west.
Replaced it with a Duramax in 2013 and you can’t wipe the smile off my face as I pull our 16k 5er now with ease across the Rockies or wherever I want to go. Get 10-11mpg and even pass gas’s stations now. Haven’t seen any comments on the exhaust brake on diesel’s, that’s worth the price of admission by itself in my opinion.