We always run in 5th when towing a large 5th wheel, TT, or hauling a large, in-bed camper with our '16 Ram Cummins. Made a number of MPG calculations running in 5th and 6th while under heavy load. Very close, if not identical MPG's. By running in 5th, our RPM's hover around 1,800 RPM at 60-65 mpg. The engine (and engine brake) is much more responsive in 5th and there's no downshifting on mountain passes. If for no other reason, we'd keep it in 5th just to reduce wear and tear to the tranny (no downshifts).
