Santee State Park is a few miles off I-95 at Santee, SC, on Lake Marion. It's off Exit 98. Spaces for large RV's are limited so call ahead. Southbound on I-26 you can take US 301 to Santee (Exit 154B) as a shortcut. Turn on US 15 to go to Santee. If you stay on US 301 you wind up on I-95 North. (If you do miss US 15, no harm done, just take the next exit which is for Santee.)
Don't eat at the Lonestar BBQ place in the old store at the turnoff to the park. It is an interesting building, etc., but the food as poor. Go to Maurice's BBQ in Santee instead.
Note: There is a sign on US 301 just past US 176 directing you to the park, but it's a back way, saves little distance, and going through Santee is a better road.