Weight Distribution, Trailer Weight and MPG

I am on my third toy hauler since 2008, 2 bumper pulls and my current fifth wheel. My current 08 Duramax has hauled all 3. I have only ever seen that big of a swing in mileage based on wind, elevation or speed. I track every tank by hand with an app and have since I bought this pickup. When towing completely empty or weigh a full load of over 2,000 pounds, my mileage isn't drastically different. Now there is differences in handling, my last small toy hauler would bounce around more when full of water, for example. But I can't really see a mileage difference with the weight, especially not 4 mpg. Same with WD hitch, my pickup was noticeably more stable with the hitch setup, but no change in mileage on my experience. If anything I would think it would get better mileage with it.

The biggest factor in my mileage is speed, and that can cause a 2-3 mpg difference. At 55 I get over 12, at 70 I get under 10.