GM Mileage 6.6 gas & Diesel

I've had three Chevy Duramax Diesel Duallies, 2005, 2011, & 2018. The 2005 pulled two TT, a 24', then a 30', then a 35' Fifthwheel. Saw very little mileage difference between them. The 2011 pulled the 35' 5er, then a 40' 5er then a 43' 5er. Again very little difference in mileage. The 2018 has only pulled the 43', but mileage is still pretty much the same with all combinations, leading to my thinking that it's the big square box behind that effects the mileage much more than the weight, since the 24' TT was probably less than 5000 Lbs while the 43' 5er is close to 20K Lbs. Overall with all three TVs and the different RVs typical mileage is 9.5 to 11 depending on conditions.
Your experience may differ, but that's what I've come up with in over 100K miles and 12 years towing.