I took the truck in to the dealer for its first oil change, and since then i'm getting about 1 mile per gallon less, in dash display. Any idea's what would cause this?

 

That doesn't really mean anything and they may have reset it.

 

Have you check it by hand? I have no faith in the dash computer since it never matches my actual mileage.

 

This. Mine has been as far off as 2.28 MPG (high) and .78 (low). Average is .66 (high).

 

^x2

 

The computer is probably compensating for the fuel that got siphoned out while it was there.

 

beige,
Not sure where you live or when exactly you took the truck in for the oil change but if you live in one of the northern states or Canada then it is normal to start to see reduced mileage as the outside temps get cooler and the air gets denser. Wait till Jan or Feb the mileage will be re-donk-u-less. As spring rolls around the mileage will begin to improve again.
Just my 2 cents.