Well you could trace the canister lines, But if you have that many miles, and it sits a lot of the time and your getting poor gad milage, I would be willing to bet you have a leaking injector or two. or if any part of your fuel lines are rubber, they are either cracked or hard so that they only leak when it has the pressure of the pump behind it. and then evaperates. A canister vapor line isn't gonna play with MPG.
When vehicles sit for long periods of time without being ran, seals, o-rings, and rubber lines start to harden and leak... thats why they say that if your vehicle sits for more than a week, just go start it and let it run for a few mins. Just so things get lubed and stay plyable.

Just a thought, but it is still a very good practice!