If you use the Patriots Point Ferry Port there is a large open lot. Parking is $5. Make sure if you pre purchase ferry tickets you chose the port you want.
You have to ride over there on a ferry. From what I saw, there is nowhere to park anything like that to catch the ferry. The ferry leaves from a very populated area of Charleston. You could always call to double-check though! Good luck! Edited to add: you may not even be able to get an RV that size through the streets of this area of Charleston.
The parking garages are free to use and they are actually within walking distance of the docks you load on
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