Knowing your rear gears and total weight would help in guessing this right, but I'd bet 6 mpg is optimistic and running 60 mph impossible.
Your gearing was chosen to get a very heavy load moving, not to move an empty truck fast.
I don't recall how hilly the drive form Reno to Elko is. I'm thinking you will be going down in elevation, but if you are climbing much, that will be slow going too.
300 miles? At 5 mpg you'll burn 60 gallons. If it only has a stock tank you'll be stopping a few times for fuel and coffee...
