Hey, I live on Holden Beach.
Not sure about Southport, can't think of a CG there. Closest to Southport is Oak Island CG http://www.oakislandcampground.com/area-info/ Not sure if it is more of a CG or a trailer park / CG, never been there.
There is a new CG ( last 5 years ) in the Holden Beach area. It is called Ocean Breeze http://oceanbreezefamilycampground.com/coppermine/displayimage.php?album=4&pid=169#top_display_media . I have not been there in the last year or so but did ride through a couple of times after it opened. Appears to be a nice place. It is about 1 mile from the beach. There is also the Holden Beach fishing pier / CG. It is the only oceanfront CG in SE NC. I drive by it almost daily and to be honest it is not impressive to me. VERY tight sites, especially when the season is in full swing.
There is also Carolina Beach State Park which is about a 1 hour ferry ride across the Cape Fear River. It is on the Wilmington side of the river.
Hope this helps some....