Just got back an hour ago from a weekend camping trip towing with my 6.4L Hemi. The trip was a short one, from Rapid City to the Custer State Park Game Lodge campground and back. I got 10.2mpg average, based on the lie-o-meter, which actually measures out pretty accurate on my truck compared to hand-calc. Although I had a tailwind for the last 16 miles which pushed that average up higher than normal. 9.5 would be more typical for this trip without that tailwind. 8.5-9.0 more typical for trips to higher parts of the Hills, like the Hill City area.
Then again, I'm towing something more modest, a 33foot TT, dry weight 6400lbs, GVWR 9600lbs, about 7500lbs ready to camp.
We've got a little bit of elevation here, about 3200ft, and the climb into the Black Hills takes us from 4500-5200, depending on destination. Nothing dramatic. At my TT weight, I've never struggled towing up 6-7% grades. Never watched the temp climb, engine or transmission. Very rarely do I need to turn more than 3500rpm.
I guess it all comes down to what you need to tow, and where you need to tow it. For my needs, the 6.4L Hemi has been awesome. If I had a 12k fiver, well then, I'd probably have a diesel.