It's funny my 03 had a similar problem when I first bought it. I could smell gas fumes toward the front of the truck. Raised the hood, couldn't see anything amiss, closed the hood, ignored it. After a couple of days it started to really concern me so I looked again Raised the hood, looked for the charcoal canister, located it on the driver side inner fender. Then I noticed the main line that carries vapors from the gas tank was unplugged, plugged it in, no more gas smell, done deal.

If I were you, I would start at the canister and work towards the tank.