The line on top frame

 

That's the vent line for the fuel tanks. Crosses over at the back of the cab, goes to the charcoal canister.

 

Should I worry about it having a hole in it? I don't smell gas or anything. Would it hurt mpg?

 

Not a mpg thing. But it vents fumes from the tank and relieves pressure (expansion). Then the purge valve sends them to the motor which burns the fumes, so basically a good thing.
You won't get anything for fumes coming out in the colder weather, the real hot days it gets more noticeable.
It's just a steel tube that goes to a rubber hose, I'd cut the metal tube back behind the hole and put on a longer hose - 2, 3 $ fix.
Ford wouldn't have spent the money to install that on every truck if it wasn't important.

 

This trucks so rusty there's probably several more holes in other places so I'll have to inspect the entire line