Tank vapors should not build up, but should vent into the charcoal canister. You may want to check the vapor lines at rollover valve / fuel fill area. These are what mine looked like last year.
Attachment 48921
That's the hose going from the rollover valve to the hard vapor line on the frame. There is also a short hose that connects the fuel filler neck to the rollover valve. IIRC early versions of this hose are known to go bad.
You have to pull the inspection plate just below the fuel fill to inspect these.