Im a new duramax owner and new to the forum. My 2008 2500HD is a crew cab with a 6.5 foot bed (truck is completely stock) that I bought to tow a 33 foot fifth wheel. I also use it for my daily commute. It has 78k miles. I get approx 16 mpg (hwy) @ 65 mph and maybe 11 towing the trailer at the...