SC and GA state parks offer a 50% discount in January and February for those that pay for a month at a time, dropping the price as low as $300 for the month. Some of the sites have full hookups which make it very nice.
If you want to boondock, there is the Francis Marion NF just north of Charleston. All you have to do is visit the Sewee Visitor Center on Hwy 17 or the main office on Steed Road to fill out a form to let them know the general area of your stay. It's deer season until Jan 1 so wouldn't stay in the forest until after then. If you want to do this, send me a pm and I can give you a couple of neat places to stay.