Gas cap. I've replaced 3 in 117k miles. There are "fixes" listed elsewhere but can't comment on them. Stick your nose close to the cap and hit the throttle a few times, you should get a good whiff of gas.
Guess I've been lucky...Original gas cap & seal @ 85k.
I wonder if garage storage has anything to do with it?
And, oh yeah. Only 1 petcock diaphragm replacement, but only due to my preventative maintenance retentativeness.