I've sailed the fantasy twice and stayed in Charleston both times parking at the cruise port. I found the parking price reasonable and less than if I had parked at the hotel . Of course I was at the Mills House, not a cheap place and the parking is in the general parking lot. If you have some time in Charleston there are great things to do. It's a wonderful city. Take a horse carrage tour, very nice. If you can take a boat ride to ft Sumpter do that. There was a great Fish Restrauant I wish I could remember it.
I'm sure you will love the Fantasy. A smallish ship but all the more fun for it. Have a blast and don't miss the big production shows. Be on deck when you leave port and see the battleship docked across the bay.