Be smart and rent a dually from McDonald truck rentals for pulling a 38 footer. Cheaper than paying for the depreciation/insurance/maintenance on a used dually. I believe your truck is rated to pull about 7000 lbs w/ 3.73. The 4.10 is not going to get you to 12k only a 2500 w/ 8.1 and a 4.10 which I have with in my Avalanche. Plus the brakes and transmissions are much heavier on a 2500.
Alternative, upgrade the lease to 2500 and see what the "hit" would be.