On our recent trip across country, we used
www.rvparkreviews.com to get an idea of what the campgrounds might be like. It was generally pretty close, but it's not foolproof as we learned in our final 2 nights. But in 2 1/2 weeks, that's still not a bad batting average. I figure it's only as good as the ratings people enter into the system, so I've tried to be good about rating the campgrounds we've visited.
Given that you're in the DC area, one campground we stayed at that we really liked was
www.lurayresort.com over by Luray Caverns. We also stayed in Bull Run Regional Park (my sister lives in Clifton) which was OK but not quite as nice. I liked the setting of Luray RV Resort better, but then it's hard to have an ugly view when you're in the Shenandoah's. It was a nice basecamp for the caverns (and Skyline Drive, although we didn't have the time to explore like we wanted).