Had the same problem, and it was the gas cap. The old one was easy to twist/click and turned yellow on the threads. Replaced it with Stant from Wallyworld for $5, and the smell went away.
If you use the auto parts stores, get the 433730 radiator. Don't get the 433716 like I did at Oreillys 3 1/2 years ago, that one is for non a/c.
For a desert truck, I'd run nothing less than a champion 3 row.