1974 Ford F250 Camper Special. 2 gas tanks, both full. Aluminum camper
shell, Approx. 150 Lbs. Stopped at a scale, came in at 5150 Lbs. No wonder I'm at 10.5 MPG. highway.

 

I also have a 1974 F250 Camper Special with a 390, but only have one tank running for now and have removed the side mount front tank. I'm usually at around 4800 lbs when I go to the waste transfer station and get weighed on the way out, including my weight and one of my kids usually for a total of around 275-300 lbs of passenger weight.

Really, I'm surprised you get 10.5 mpg on the highway. That's where I tend to get my worst fuel economy. I get around 10-11 mpg driving around town and on highways where the speed is 50 mph or less. I can rarely tell the difference in fuel consumption regardless of the kind of speed or traffic when driving below 50 mph. Once I get up to 60 mph, my fuel economy takes a pretty serious nose dive to maybe 7-9 mpg. I have a 3.73 rear end, so maybe you have 3.50 or so?

 

Sounds like the eco dentside model

Still not bad for a 2.5 ton wall of steel.

5700# 8 mpg (downhill)

 

5511 pounds at the local gravel yard on their scales. 1978 F250 4x4 with 351M. 2 people in the cab.