When my light comes on it is all I can do to put in 20 gallons. If yours does that, 300 miles would be 15mpg. A bit low, but since it is new it could come up.
Who filled it up the first time? If it was the dealer, they likely shorted you a bit by not topping off.
Hope it gets better, but if it doesn't, it is still a Hummer.
