There is another approach to this problem that might be less expensive. You did not mention what kind of trailer you are pulling but perhaps getting a lighter one (like LivinLite) or a lighter and lower lower one (like Trailmanor) might be a better option. That way you keep your existing truck. For what its worth, I have a 3124 Trailmanor (opens up to 31 feet long) and I pull it with a 2005 Avalanche 1500 with a 3.42 rear axle and I get around 16 mpg. With gas prices going back up MPG for all of us will become a sensitive issue.
Grrr: Just re-read your OP and saw that you already bought the new truck....