Here's the link to the
Metro Home Page. Depending upon where you park outside of D.C., you can either use just the train or a bus and train. As was mentioned, Cherry Hill Park is an excellent RV Park with very helpful staff to answer your questions. The bus to the rail station stops at Cherry Hill Park. The Metro can seem very confusing to newbies, but once you figure out where you are and where you want to go, it's not too bad.
I would also recommend a trip to the National Zoo in D.C.. The Washington Monument, Lincoln Memorial and other memorials are nice too.
We drove into D.C. from Cherry Hill Park on a weekend and were able to find parking, but it took a bit of searching. The traffic wasn't too bad. But during the week, use the Metro.
Have a great trip!
~Rick