Ya there's about 1000 mpg threads on here, but I'll play.
If you're getting 12-13 with your diesel, I'd expect 8.5-10 with the 6.2 gasser.
Have had 2 of them ford 6.2s now and empty (800lbs or so in the bed) mpg was 12-13.5. Loading the bed and pulling a smallish trailer was around 9.
Comparing similar loads with my diesel vs most gas engines is about a 4 mpg penalty.
Oh and don't get sucked into the gvw game. Your 2500 chebbie is the same truck as a 3500 minus some spring capacity. And tires have nothing to do with it. You can buy heavier tires, unless that's the justification for a new truck.