You need to make sure that the tank and it's filler are in good condition.
You didn't list the truck's year, but it is probably at least 35 years old. The rubber filler connector is prone to cracking and leakage, the seal on the cab that keeps the filler neck opening outside the vehicle, the fuel line running to the motor, the seal for the sending unit, all dry out and can allow fuel vapors to seep into the cab.
The tank itself can leak because of age.
Ask yourself some isolating questions. Like; " Does it smell ALL the time?" A yes there would require a through check of everything listed above. or " Does it seem to smell more on right turns ?" ( opposite side of filler.) Check all the rubber seals for the filler neck... "Is there gas in the cab under the tank?" RUN for it !!!
Track that smell down ASAP. Not only are gas fumes not very good to breath, but gas vapors are HIGHLY explosive. Best of luck.
DVal