We were there this past October. Most RV parks around the DC are very expensive ($40 and up). We found Prince William Travel Trailer Village in Dumfries, VA for $25 per night with full hookups. Really nice park about 30 miles south of DC on I-95. It's a short distance from the freeway. It's listed in most campground directories. It's 15 miles from the nearest Metro sation. As far as I was concerned, the Metro was the only way to go. Traffic is really bad during rush hour. The one cloest to Dumfries had pleanty of parking. The staff at the RV park was very helpful with directions to attractions in the DC area.
2003 Alpenlite Valhalla, 29' RK 2005 Ford F250 PSD