Is there a fuel cap opening tool?

[redars]

If the caps are too difficult to open , then they require adjustment. It is a very simple procedure.  Remove the cotter pin, loosen the nut. Test to see that it is easier but still tight enough to seal.  Upon completion of the test , install a new cotter pin.  
 I never let the fuel personnel put the caps back on.  If they get cocked , it makes them difficult to open again, even if properly adjusted. 

[redars]

See Mooney SB M20-229A