Take 124 to 87 then 3005 to 257 then 332 at Freeport. Good roads for most any size RV. At Freeport you have to get off the barrier islands. Allow at least a few days to explore Galveston Island. I believe 87 south of Port Arthur is closed and if heading south you have to access it from 124.
If continuing south you can get back on the barrier islands at Port Aransas to Corpus. Allow time to explore the Rockport area. Then don't miss the historic aircraft carrier the Lexington at Corpus Christie. Lots of great secluded beaches along the way too. Goose Island State Park, Galveston Island State Park and Mustang Island State Park. Also Padre Island National Seashore. A side trip to Goliad State Park is also worth it.