Even with the 8 speed and 3.21 ratio, with 35" tires its not going to tow well at all. your looking at an effective ratio with the larger tires of around 2.90. that is not going to work to your advantage. and yes gas mileage will be horrible (I`m guessing around 5mpg, 6 at best!) with the small fuel tank you will be limited on distance. I wouldn't want that.
As far as trailer weights. always add approx. 1200lbs to the dry weight. so 6600lbs dry will weigh in around 7800lbs. this will give you a loaded tongue weight of around 1100lbs. and don't forget to add in the weight of the WD hitch.
As far as I`m concerned its a no-go. either return the truck back to stock or trade up. your to far away from even being able to try it out to see if you like camping. in fact you aren't going to like the drive while towing the trailer.