I've got a 2005 F350 diesel long bed with a 38 gallon fuel tank
and 37,000 miles.

Yesterday I encountered a situation where the engine started
acting like it was running out of fuel.  I was pulling my 13,000
pound 5th wheel trailer over highway US-70 between Roswell
and Alamogordo, New Mexico, starting the day in Caynon,
Texas.  I frquently will drive until the low fuel light comes on,
whereupon I will fill the tank full with 30 to 31 gallons of diesel.
So a couple of miles after I cross over the top of the pass the
Low Fuel light comes one.  I think to myself, fine, I've got 7 - 8
gallons left, 35 miles to my destination with a elevation drop
of 3000 feet.  No problem.  I usually get 10mpg pulling the
trailer on flat ground.  Coasting down the pass could be
better than 20.

I get to 15 miles from my destination, within 200 yards of the
bottom of the pass, and the engine starts to act like its out
of fuel.  I ignore it for 30 seconds and then it does it again.
So I start to be real concerned, find the closest parking
place, drop off the trailer, and drive 1 mile back to buy
some diesel.  Turns out, I could only put 33 gallons in the
tank.

So what was happening here?  Was I really just about out
of fuel?  Or was the fuel pump sucking air because I was
going down hill?  Most of the pass was about a 5 to 6 per
cent grade but the last 2 - 3 miles was probably a 2% grade.
Is the tank not really 38 gallons?  The most I've ever put in
was 32.5 gallons.  I suspect that driving down hill was the
problem and that if I just continued on to my destination, I
would have made it no problem.