The rear axle ratio matters a lot on these - my 2019 DT with 5.7 3.92 ratio and towing 28' camper does 12mpg at 65-70 mpg. Once you push to 70+ the MPG drops like a rock. First time hauling the camper at 75-80 it was 7-8 MPG. I just slowed down and found that sweet spot where it will stay in 8th with occasional downshift to 7th to climb.
The rear axle won't matter in terms of MPG; both trucks will end up pulling the same weight at the same speed at approx the same RPM (they will be in different gears in the transmission but that doesn't matter, what matters is RPM + weight + frontal area + mph).
Probably the two biggest variables in towing MPG is speed and frontal area. You can have a 20 foot 3000 pound (empty) trailer or a 28 foot 6000 pound (loaded) trailer, but if they share the same approx frontal area you're basically plowing air and going to get in the neighbourhood of 8 to 12 mpg. The faster you go, the more the frontal section matters.
You may also want to re-consider towing your camper in 8th gear, that's hard on the transmission (and probably not good to lug the hemi either), direct drive (6th) is best in terms of least stress.
So for my towing I always focus on RPMs and never on speed. I prefer to tow at 2300 RPMs on flat ground, and never tow at < 2000 RPMs, I'll use my gear limiter to force this if necessary. My MPG is between 8 and 11, depending on head wind or tail wind.