Diagnosis by elimination (po442)

Question here:

1 month ago, check engine light came on. Not surprised, actually, because the truck had started to run VERY roughly. Service department recommended a full fuel system treatment including changing fuel filter, additives, etc. Tab came to about $300...a little steep, but the truck was running noticeably smoother.

2 weeks later...engine light came back on. Code: PO 422 (small evap system leak). I assumed that this might be related to the service. Took truck back to service department and they're puzzled. They spent 2.5 hours going over the system and could not find a pressure leak anywhere.

They are recommending a new evap canister (costly). I asked straight out if this suggestion is based on observed conditions, or if it's simply because everything else checks out. Bumbling answer...it's the evap canister BECAUSE everything else checks out. They said it's likely an "internal leak," but have no way to check. I understand this to be true.

So, question is: Is it a wise decision to spend $$$ on a new evap canister at this point? I would prefer to get a second opinion or at least have a mechanic offer better certainty in his diagnosis.

Thanks for the help.