I don't know if you are still interested in this info but I'll tell you what I know. I haven't stayed at any of the ones I am about to tell you about but I have heard others discuss this and most people recommend Cherry Hill.
http://www.cherryhillpark.com/index.cfm I think this recommendation is primarily based on the fact that it is the closest park to DC but everyone also says they felt safe there.
There is a KOA in Millersville, Md that is a bit further away but still close enough to drive to DC or to the metro. Also, it is closer to Annapolis and Baltimore if you were interested in visiting either of those.
If you are interested in Gettysburg, I would plan to break the trip up into 2 stays- one near DC and one in Gettysburg. There is a lot to see and do in Gettysburg and there are several nice parks there. The drive from DC to Gettysburg and back as a day trip wouldn't give you much time.
I read a fulltimer's blog of a couple who recently visited DC and they stayed at Little Bennett Regional Park. Here is their blog if you're interested. They were there to see family in DC and it sounds like they liked this park.
http://laurieandodel.blogspot.com/2010/06/visiting-family-and-friends-in-dc-area.html I thought this park looked more private and woodsy than the commercial parks plus it is closer to Gettysburg if you do decide to try to do both from the same park.
Hope this helps.