Here's the story.....
Heading west through Texas I can get anywheres from 8 to 11 depending on the constant wind at 67 mph.
Going through the Rockies I get 8 to 10 depending on speed and how often I fight before going into the truck lanes.
Going through AZ I get 11 to 12.6 depending how fast I drive anything over 67 costs and at 70 we are sucking fuel.
No load here in Michigan I do 78 to 83 and get 19 down to 16. At 73 I go 19 - 21.5
It is always relative. towing a TT at 70 can really suck fuel and just to do it really doesn't get you there much faster. I am most happy towing at 67-68 and I destination drive, one place to another whether it is 5 hours or 13 ,I still do it at 71 years old.
2018 Jayco 322 40' with addons and well over 5.5 ton.