We have stayed overnight in Walmart parking lots a few times while traveling (plus one night at a Cabelas). I do not consider it "camping." I always call the store first to confirm they allow overnight parking. I also ask where they want us to park. We pull in, level the trailer, and put out the slide (we look for a spot where the slide will be over grass, not stuck out into a lane). Most of the time we don't unhitched, but sometimes we do to get fuel for the truck, a much easier task without the trailer. We are well set up for boondocking with a stove top coffee maker (an Aero Press makes great coffee!), LED lights, big fresh/gray/black tanks, a 12 v TV and VCR, etc. As others have said, your wall sockets won't be powered. In the morning we take a shower, have coffee, pack up, and are generally gone by 8:00.
There are a few places we have seen where folks obviously stay for multiple days, but you won't find that allowed in popular tourist areas. I would be amazed if the Myrtle Beach area allowed parking lot overnights at all, but the OP may have been talking about his travel to the area.