I just find that the bigger engines will do better on mileage in those situations.
My V8 ram got 8 mpg towing with 4.11 gears, my V10 ram gets 10 mpg with 3.55 gears, actually got 11 mpg towing 250 miles with an 8000 lb trailer. I only get 11-12 mpg empty, though. Towing 65 mph @ 1700 rpm is effortless.
Buy the big block over those earlier 6.0s.