Ok to the OP, your numbers are dead on if you are running 37" tires or so. My V10 excursion would get 7.5mpg avg towing 10k with premium fuel. I made the mistake of running ethanol at 87 octane and yes, it was sub 5's. So 4.5 isn't unheard of.
To the rest, he said he bought an 2007 6.0L, they don't have any issues, it was the 03-04. I wish these guys would stop making these comments.
Or I could say
Oh hey, you bought a duramax? Hope you like the repair bill when the Bosch injectors fills the block with fuel and bends your connecting rods.