I currently have a lance 1172 with a 2015 F350 "Heavy Tow Package".
That package gives me 4.30 gearing AND the tighter turning that the 450's have. I believe I have 500 lbs more towing ability than the 450's have also.
I've heard all you guys with 373's, 355, 410 gearing. With the camper (heavy sucker) and a 22 ft Ranger Reata I average 10-11 mpg. And if the speed limit is 80, I usually do 80. Mileage does not seem to matter if the boat is attached or not. So, I guess what I'm saying is the gearing doesn't seem to matter when loaded up and towing. Don't know what empty MPG is since I keep the camper loaded but when new seemed to be 15-17 IIRC.
BTW. This Friday I'm trading my 2014 Lance 1172 for a brand new, 2018 Host Mammoth. What an awesome camper