Go through a couple tanks, recording your mileage.
Make sure you always fill it all the way to the top.
But yeah, the truck is heavy and the wheels are heavy - so it's sensitive to how you drive. It's got the power to get that 4K lb truck and those big 33" tires spinning, but be aware that doing so burns up those old-dead-decomposed-dinosaurs pretty fast.
Good point to go through a tank or two to see what is up.
plus your driving vs the other person who put the 6500 on...who knows...
doesn't the ecu learn your driving style?
best of luck with your new truck, do not throw in the towel yet...you gotta post up some pics if you can of your new to you truck!