It sounds like you may have multiple things going on.
If your MPG took a major dive thats a sign that your ac compresor may be going out. However, you can try and have the AC recharged as freon is actually what lubricates the AC unit and it may reduce some of your A/C drag.
Im guessing that your running Under Drive Pulleys? Even if you have UDP's or not in an effort to keep your engine from overheating it would be wize to make sure that your radiator fluid stays fresh. I forget which parts store has them but you can get test strips that you can use to check the PH level of your radiator fluid with when you do oil changes so you can flush the radiator if necisary. If your radiator fluid is currently good it may be time for you to switch to a cooler running thermostat. I will go back to my books and do some research on what degree thermostat you can run without problem.
As for the fuel smell you may have a leek in your fuel system somewhere and this could also be the contributor to your loss in MPG. See if you cant chase the smell down by inspecting your fuel filter, fuel rails and fuel injector seals.
Note that depending on what modifications you have and how your car is tuned this can cause the exhuast to have a fuel smell, if you think the smell is coming from your exhuast that could mean a few things as well. It could mean you have a bad spark plug, have a cylinder losing compresion, or simply need a better performance tune if you have a few bolt ons.
If you list your current mods it could help with a diagnosis of whats going on. But if I had to bet money your having a fuel issue somewhere that is robbing the engine of power and hurting your MPG and power badly and that the loss of power is making the engine not cool as easily with the possibility you need a radiator flush. For my best guess I would need to know what mods are done.