Yeah, I don't get why you checked in if you didn't agree with the rate. They are not the only hotel in Olive Branch. Now that you have made a good faith effort to contact the manager, what I do in such a dispute, is I take my copy of my reservation with the rate clearly marked, along with whatever evidence I have of my good faith attempt to communicate with Marriot (or whoever billed me wrongly, I'm not singling out Marriot here), such as copies of the emails acknowledging that you got poor service, and I would charge back the credit card. I don't know what else you can do. It's a PITA, and I usually have to mail all of my evidence certified mail, with proof of delivery, and sometimes I have to mail my chargebacks in two or three times, but eventually the credit card company will take action.
But I don't understand how it gets to this point. Nor have I experience anything quite like what you describe. Because, quite frankly, I don't think I have ever checked into a hotel that turned around and demanded a higher rate that they said in my reservation...I mean, I guess there are resort fees but not in Olive Branch!