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No experience here - but suggest to continue to wear the dealer out - this certainly isn't right.
Is the smell more like raw gas, or burned gas?
Has the PCV system been checked out?
Is the carbon canister / fuel vapor recovery system working properly?
Any fuel getting into your oil - perhaps take a sniff of the dipstick?
Fuel lines thoroughly inspected for leaks / seepage - both the supply and return lines?
Sounds like this is going to be a challenge to chase, especially since the dealer has already taken a look or two. Good luck, and keep us posted on your findings.
1989 F150 XLT standard-cab shortbed 4x4, 5.0L, AOD, 3.55 gears, manual hubs, 31x10.50 tires, 218K miles. Repainted Oct09. SOLD - Jan2015
2014 F150 XLT standard-cab shortbed 4x4, 5.0L, 6-speed auto, 3.55 gears, auto hubs, locking diff, trailer pkg, 5K miles.