I have had my truck for 8 months, it does have high miles on it; 216,000. I have been having a stronger fuel smell. It is not throwing a code, so I am not sure I need to replace the O2 sensors. I do have custom exhaust. Flowmaster series 40 I think. The PO has it cut off after the cab and welded on. What I believe is the muffler section comes has to output ports. I think it is a cat back exhaust but i'm not certain. Obviously having a 4x4 lifted 4.0 V6 is not going to give me great gas mileage. I average 13MPG per fill up, in town driving only) But I think that doing some changes could help my issue.

Does anyone else have this fuel issue with no error codes? I have not had the truck scanned, so I don't know if there could be any invisible code problems.

 

Pull a spark plug

Nice light brown patina on white electrode means good clean burns.
Blackish is rich

Plugs should be gapped at 0.054

O2 sensors the 2 upstream sensors closest to engine are rated as 100,000 mile replacement, they do tend to fail rich so if plugs are getting darker brown then your MPG will get better if you replace them.
Rear O2 "sees" cleaner exhaust being after the Cat converter so tends to last alot longer, and it is just used to test if Cat is working, not used for air/fuel mix ratios.

"Smell of gas" can mean two things?
Do you smell raw fuel or burned fuel?

The Filler hose and Vent hose on the gas tank do crank over time and you will smell gas, usually you would get an EVAP code but..............if you have never had a CEL(check engine light) since you have owned it that might not be working.
Does CEL come on with the key and then go out, it should.