10,500# fiver with the 3.73 axles - it does fine but a diesel really would be better. Mine is my daily driver 98% of the time and towing 2% so I'm fine with it. My MPG are pretty identical to what Drittal posted. Stop and go city driving kills the MPGs. As does flying down the highway over 65.
At 10,500# I feel as though I'm pretty close to maxed out. The 4.10 gears would help. Doing a gear swap to the 4.56 would really make this thing a towing beast. But again, I'm fine with my setup. The 12K you are proposing? That's getting up there but still within the theoretical capabilities of the truck.
It really is all relative. 20 years ago you'd be laughed at for suggesting a 2500 series truck could even dream of towing a 12K trailer while getting anywhere near the MPGs the 6.4 Hemi gets. Overall, this truck is outstanding.