Don't know how late you want to drive. From Myrtle, 3 hrs to New Green Acres RV Park, in Walterboro, SC is popular with snowbirds, big drive-thru sites, easy on and off I-95. About 5 hours will put you near Brunswick, Ga and IIRC, there's a Flying J in the vicinity. In Jacksonville, take I-10 West to US 301 South. 301 is 4 lane and passes through a few towns, but mostly open road and light traffic. Two particular issues with speed on US 301... Lawtey, Fl with a school zone Mon-Fri and south of Starke, Fl lies the City of Waldo, long known as a speed trap. The road is very clearly marked today, but speeds drop quickly. "Waldo Willie" has lurked there since the 70s. Just north of Ocala, I'd turn off onto Hwy 326 and head a couple of miles over to I-75 and south to Tampa.