1994 Ford F-150 gas odor and rough idle

Please help. I have a gas odor in the cab of my 1994 5.0L F-150 Supercab. Associated with the smell is a very rough idle. It is especially noticable during warmer weather with the A/C on and stopped at a light holding the truck with the power brakes.
I have replaced the plugs, ignition wires, cap, and rotor twice and the fuel filter and O2 sensor once. According to the techs that have worked on the truck, it is not throwing any codes and for whatever reason can't seem to duplicate the problem. The truck was in the shop for a month and I didn't drive it for fifteen minutes without the problem returning. It spends a great deal of time parked at the airport in Melbourne Florida exposed to high humidity, heat and salt air. Most of the parts I've had replaced had a great deal of corrosion. Any ideas?
Thanks, Gil