Assateague Island

I have not stayed there over that time, but have been there in the spring-summer and fall...

The one concern I would have ( if its even a concern? ) would be the temps and wind... with it being right on the Atlantic, the wind typically is higher, and at that time of year it would be blowing cold air... possibly

That's all fine if that's what you want, I am not so sure there are any other negatives???

First, I would check to see how the area fared after Hurricane Dorian. Know some of the Outer Banks received some real damage. Have been in that area several times around Christmas. It is usually windy (totally usual for the area) and temps can be near freezing - although there were times it was more like October than December). Not at all sure campgrounds in that area would be open in December. Perhaps check rvparkreviews.com for names and contact info in the area.

Never stayed there in December, but did in October. It was chilly for sure. But you can have some nice sunny days too. The State Park is the only one with electric and they only have one loop. Some of the loops in the NP allow generators.

First reaction...Brrrrrrrr! Been there late October & had it cold then. Never been in the SP. Always the National Seashore area & always on the bayside loops which are much more sheltered from the wind.

If you are determined to do some Christmas camping on the Delmarva I would be much more inclined to use one of the inland & sheltered parks like Killens Pond SP, Martinak SP or Tuckahoe SP.

We spend much of the winter months on the Oregon Coast very close to the ocean. Yes, it can be cool in the winter, but you have a trailer for shelter.

Coming from New Hampshire I'm sure you understand cold weather. Although December cold in MD maybe warm compared to New Hampshire in December.
Other than the cold it's doable. Not sure I'd want to do it, but if you a feeling adventurous go for it!