I will offer two options and recommend you do a virtual drive on Google Maps of both to figure out which you prefer.
Take James Island Connector (hwy 30) and turn left on Folly Road. Right on Camp Road. Right on Riverland Drive. Left into the park. Biggest problem with this route is the right turn onto Camp Road as it would be tight.
Take James Island Connector (hwy 30) and turn left on Folly Road. Right on George L Griffith Road. Right on Riverland Road. Left into park.
I do not recommend using Central Park Rd because of the left onto Riverland which is usually bad enough in a car but can be done with RVs as I see them all the time, just takes patience. Also will warn that at above normal high tides, the road does flood with salt water but not to the point it can't be driven.
Riverland Dr has a bad reputation which is deserved to the drivers not paying attention. The old large oak trees have the scars to prove it. However, if you pay attention, don't ride the white line and you will be fine.
2015 Ram 3500 4x4 Crew Cab SRW 6.4 Hemi LB 3.73 (12.4 hand calc avg mpg after 92,000 miles with camper)
2004 Lance 815 (prev: 2004 FW 35'; 1994 TT 30'; Tents)