I have a 6.0L 2500HD pulling a Jayco 284BHSW(6000-7500lbs). Drove to Michigan last week and back from Columbus Ohio. I got about 8.5mpg with a lot of stop and go traffic in Michigan thanks to construction zones. When I pull my landscape trailers(open air) of similar weight but less wind resistance in the city I usually do better, maybe 9.5mpg. No trailer in the city 11ish, no trailer road trip around 15mpg. I think the wind is a huge factor obviously. Would love to make myself more aerodynamic while towing. I imagine 5th wheels would be worse.