I have had decreased gas mileage used to be 17 MPG city, now average 12.8 MPG city. There is a faint smell of gasoline sometimes and I have changed the gas cap to an OEM branded one. The AutoZone scan tool suggested a Natural Vacuum Leak Assembly and when looking on Amazon a Purge Valve was the most frequently purchased item along with a gas cap. My question is would this cover all bases or should I be doing a smoke test? Any other Ideas?

Codes from scan tool: P0455, P0456, P0457, P0420