If nothing has ever been serviced with the gas tank pump or EVAP system then it's way past due for an inspection. You will likely find that a lot of it needs replacing. This will require a tank drop so you can inspect the pump for leaking and corrosion unless you can snake some camera in there. My bet is the pump is leaking.
About 3 years ago the previous owner sold me my 05 Tahoe with the gas smell problem. I didn't think anything of it. Just thought it was an EVAP issue. Boy was I wrong. It had a really bad persistent gas smell. Seemed to get worse, especially when it was in the garage. Seeing how it could possibly be serious I decided to play it safe and dropped the gas tank. I found that the pump was rusted (just like the picture shows). I ended up cleaning the tank, installed a new Bosch pump, bought new gas tank straps, and then replaced most of the EVAP related parts. That includes replacing the carbon canister, purge and vent solenoids and new nylon lines that tie it all together..
