OK I did the numbers and slowing down from 70 mph to 65 mph, saved 3/10 of a mpg.
My tip last year running right around 70 mph gave me 9.5 mpg one fill up (Both tanks) and 9.6 the other.
This trip was 1049 miles on 107 gallons. Which worked out to be 9.8 mpg. All but about 30 miles were hooked to the fiver towing.
Now a "Full load of fuel" in both tanks is 125 gallons or so. So this trip was one fill up. Last years trip was a bit longer (1,600 miles) thus required two fuelings.
Now another varible was this year I had the fresh water tank full as I planned on a few days without being at a campground. So that is around 80 more gallons of water than I had last year. Last year I had maybe 20 gallons of water on board. This year the 110 gallon water tank was full.
So instead of the 23,000 pounds I scaled last year, I scaled almost 24,000 pounds this year.
The rig/combo is the one in my sig.
That's the facts Jack!