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Frontier Town and Ft Whaley (Sister Parks about 10 miles from each other) - http://www.frontiertown.com/
Castaways RV Park - http://www.castawaysrvoc.com/
Assateague Island State Park (limited electric sites available and no site water/sewer hookups) - http://www.dnr.state.md.us/publiclands/eastern/assateague.html
Assateague Island National Seashore (boondocking only) - www.assateagueisland.com/assateague_camping.htm
I have stayed at all of the above places and they are very nice. There are many more campgrounds in the area that are a little farther away, just did a google search for Ocean City Maryland campgrounds.
Frontiertown is well maintained, friendly but a bit expensive. The only down part is the golf carts. The kids tend to go crazy in them during the day, and they aren't well policed.
It is on the mainland, but a free shuttle bus runs to the boardwalk all day until I think 11 pm.
Other then that one thing, one of the nicest places I've been to.
Ft Whaley is a little bit less expensive then Frontier Town (I guess because it is farther away from the beach, only a few miles though). Ft Whaley also has a shuttle bus to OC and also a shuttle bus to and from Frontier Town, so you can use the waterpark.
If you don't mind driving a bit, we stayed at the Rehobath state park on coastal highway. about 20 mins north but only 38 a night for full hook ups. nice campground clean, flat, right on the water!