Don't know about Myrtle Beach, but your best bet while visiting the outer banks is Colington Park Campground in Kill Devil Hills, NC [(252)441-6128]. My husband, son and I stayed there for a week last summer, and it was great.
It was the only campground we saw that had any kind of shade. It's a mile off the main drag and that same distance from the beach and the Wright Brothers Memorial. They have electric and water hookups, and the office has a small store if you run out of anything. Best of all: it's a really peaceful setting in the middle of a very commercial area.
It was also the cheapest place we found, running about $20/night (last year, anyway). I would recommend getting reservations. If I remember correctly you will have to pay a portion up front to reserve your site.
Hope you enjoy your trip as much as we did.