A couple of months back I got a check engine light for a couple of days out of nowhere on my '14 350 FSport. By the time I scheduled an appointment at the dealer the warning went away so I cancelled the appt.
But I have noticed that since then the fuel economy is terrible. What could this mean?

 

See if there are any codes stored.

 

Fuel economy can be effected by so many things...weather, change of winter fuel to summer fuel, driving habits, tire pressure, etc.
It may still be possible to retrieve the code(s) that triggered the check engine light if they are still stored as History codes in the ECM.
It might still be worth getting it checked.

 

Any recommendations on a code scanner?

 

Also forgot to mention that after disconnecting the battery for a couple of days the fuel economy is how it was when the car was new but then slowly degrades again.

 

Crappy Tire has them on sale this week. Although you would have to check the box and make sure it shows history codes...not all do.
Although if you knew someone with Techstream software that could check it for you then that would be the best as you can see the history codes...if they haven't been cleared by disconnecting the battery.
That process erases all the learned values, so basically it reset the ECM to start learning fuel trim again.
Might just be something like an O2 or A/F sensor that's getting old and lazy...it happens...to people too I heard lol