2015 Sierra 1500 options

I am thinking about upgrading my current tow vehicle, a 2010 Sierra, to a new 2015 Sierra. My current and new truck are 4X4 crew cabs. Current truck has the 5.3 with 3.08 gears, towing with this truck is listed at 6200 lbs. Does well with my 28Bh Grey Wolf that is in the 4800 lb range. We are looking at going to a bigger camper, still a bumper pull. My new camper would be no more than 8000 lbs, more like 6500 lbs. I make 2 big pulls a year, each are around 800 miles and a few short pulls. That being said gas mileage is important because the truck is a daily driver. My options are the 5.3 with 3.42 gears, standard truck rated at 9100 lbs towing. The 5.3 max trailer package with 3.73 gears rated at 10800 lbs. The 6.2 with 3.42 gears rated at 11800 lbs. Trying to stay away from the 6.2 due to the extra price of the truck, around $4000. My biggest concern is the fuel mileage between the 3.73 and 3.42 gears. I thing the max package with the 3.73 would give me all the towing I need but how will the mileage be. All replies are appreciated. Thanks