It could easily be either one, or a leak in a different component. They're not that difficult to test. The canister vent valve is normally open, but should close with 12v applied; test by removing it from the truck and trying to blow through it with a short hose attached, with and without 12v to the terminals (repeat to verify that it's not intermittent). Purge valve is the opposite: normally closed, but opens with 12v applied, and it helps to use a vacuum gauge on that one. Many YouTubes on how to test using various methods.
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