Very Slim Pickings in Baltimore County
Morris Meadows in Freeland (off I-83 near PA line) is the only one I know of in Baltimore County.
http://www.morrismeadows.us/
Ramblin Pines is in Woodbine, Howard County (West of Baltimore, Near I-70 and MD 97)
http://www.ramblinpines.com/
Capitol KOA in Millersville, AA County (South of Baltimore, Near I-97 and MD32)
http://www.capitolkoa.com/
Bar Harbor RV Park in Abingdon, Harford County (Northeast, Off I-95)
http://www.barharborrvpark.com/
That pretty well covers the Baltimore Metro Area. The Millersville KOA is only one I've stayed at. Nice, but some sites are small in the wooded areas.
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2004 Dodge Ram 2500 Club Cab 4x4 CTD
2007 Sunline Solaris T-276-SR: Misty Blue/Cherry
2001 Dodge Ram 1500 Reg Cab 4x4 OFF-ROAD