There is a Walmart in Pocomoke City,MD. Not sure what their policy might be. Not near the beach though. Easy trip out to the Virginia side of Assateague island. However,they will fill up early on a holiday,like by 10 am.
There are also state parks nearby. There are campgrounds on Chincoteague,VA but still not oceanfront.. they probably have a 3 night requirement because of the holiday. There is also Assateague,MD State Park. Just electric but literally right over the dunes to the ocean.
I assume you are coming up 13. There is a MD welcome center but I d ont know what their overnight policy is. 13has a lot of truck traffic but I don?t know what truck stops,if any,are available.
Nothing else comes to mind but if you have questions,I may be able to help.