I'm getting a gas smell after I let off the throttle getting off the highway. The only things I have done lately are change the PCV valve and grommet, an oil change, and a fuel filter change. The air filter is pretty clean, as is the throttle body.

 

Time to find that leak inside your intake. Search on CPI INJECTOR and learn how to search out the leak. theres not really any other way to be smelling fuel. Milage getting worse?

Might help if we knew what you were driving, Im just making the obvious guess as those injectors are a huge problem with these trucks 15 years old.

If this is the engine you have, then open her up and look for the leak.