If you had any experience towing you wouldn't be asking the question, so get the newest dually you can afford. And personally I wouldn't even think of towing that much long distances without it being a diesel.
$15k will get you a nice hopped up 2nd gen cummins Dodge. Look for a tuck with the 6 speed manual and enough go fast parts to make it pull like the newer trucks. That would be my choice on a budget. Or find the nicest early Duramax you can for the budget.
7.3 early super duty with 6 speed with some mods would be good too, but they're getting more scarce than the other 2 brands due to Fords 8 year hiatus from making good diesel pickups.
You'll have to shop hard for a deal and know what you're looking at to not get a clapped out older diesel as most all will be higher miles or still $20k for a dually 4x4.
2wd will save you a lot of money and potential repair costs as well.
