When you are traveling 55 empty what are you taching and what about when doing 85 mph??
When I first got my truck 1993 f250 xlt 4X4 with auto locking hubs with 185,000 miles on a 7.3 idi natural aspiration, it now has 264,000 miles on it, on it I drove it to Florida from NJ and average 100 mph, seriously, and tacked out under 3000 rpm maybe around 2700 rpm and at 85 mph I tach 2200 rpm and I got about at least 22 mph. I started with two full tanks of "fuel" one tan is , think, is 18 gallon and the other is 19.7 gal. I fueled up in the carolinas and then in Georgia just before Florida and drove around Florida for ten days and got back to Georgia and filled up again and again in the Carolinas and made it to Atco NJ by the race track and yes used the air conditioning
I also have a '94 f super duty dump truck with a 7.3 idi and natural aspiration but that truck is geared so low that when I am going 55 mph I tach 2200 rpm and loaded with about 4 or 5 ton I get 8 mpg and empty I get 12 mpg. Both of these trucks have air conditioning that never gets shut off. Has a lot to do with the gears, both the above trucks have automatic transmission that have been rebuilt with steel plates and 6 pins steel instead of the aluminum ones , I had them rebuilt heavy duty as I plow with the 450 and the 250 has a shift kit in it where it shifted once from 1st to 2nd and broke the yoke off the drive shaft
There are two different class engines in the two trucks as the 250 is just a regular diesel and the 450 super duty has a diesel made heavier duty as is in a bigger truck and I looked at the engine and is stamped that is a heavier duty motor.....not sure if any of this helps but that is my situation

 

I was recently delighted to see that following my fuel return line replacement, I got 18.3 mpg. This was about 50/50 city and ascend/descending 8k elevation. A little 4wd in the snow. Kept it around 1800 rpm ascending and 1600 rpm descending. Yes, with the camper. 72k miles

 

That is impressive. The 5 speed is much better with fuel than the e4od. My e4od idi truck got about 12.0mpg with a 6-pack camper, and about 14 without.