Wish I could help. But I have no Shade Tree experience with the EVAPS, shy of replacing the gas cap. I have an idea where the vent valve is but mine, if that is what it is, looks like it had been burned by a touch.
What I can tell you though is that if there is a vacuum line going to that vent valve and it is leaking, that would definitely affect your gas mileage. But it would also affect how the engine idles. Making you Idle rough and unsteady.
A few of the members here have worked on their Evaps system. Hopefully one of them will chime in and maybe even have pics. I would like to see it too. So i will know if that is what the Burned part with an electrical harness and a vacuum line that does go over to the Evaps canister is.