First things first - did you keep an email confirmation when you booked? Was resort fee in the email confirmation? If not, fight the fee.
Call the hotel. Point out that there was no resort fee when you booked & you have written documentation. Tell them you expect them to waive the fee. They should do the notorious 1-time exception for you. Push hard back if they argue w/ you. Tell them (only if they won't give) that you're willing to push this up the AG ladder, in addition to CC company.
Cheers.