To vent, or not to vent (gas cap)

[micoviso]

When I bought my 1993 Fleetwood Coronado (Chevy P32 454) last year, the previous owner told me that he had put a locking gas cap on to prevent a neighbor from stealing his gas. I like the idea of a locking gas cap, but it might be causing me some issues. I notice that after driving for a while, when I remove the gas cap, there is a noticeable vacuum sound, so obviously, this is not a vented cap. Should it be? I've had one instance of vapor lock on a particularly hot day driving through hills in south/North Carolina. Also, my generator has shut down while driving, and wouldn't start again until the RV sat for a while not running. Does anyone know if the original gas cap on this RV was vented, or is that vacuum supposed to be there?

[DENSHER]

Just went to NAPA online and chose Chevrolet Truck P30. It comes up with non-vented cap. Even non-vented does have a vent only difference is the non-vented has a check valve in it that will open to allow air into tank to prevent vacuum and closes to allow build up of pressure.
Sounds like check valve in your cap is stuck closed.