CG's are your only option in the DC area. Here is some options for you.
Best cost wise and shortest commute to trains would be a monthly site at the KOA in Harpers Ferry West VA and use the Brunswick Marc Train to get to DC. Current Monthly Train Ticket costs from Brunswick to Union Station is about $250, From Frederick to Union Station is $216. So you can factor in these cost in when looking at your options. If you would be getting off at the Rockville or Silver Spring stations ticket cost would be lower. You can calculate them on the Marc Train Web site.
KOA is open year round if you need to be here thru the winter. You can connect to Metro from the MARC Trains at Rockville or Silver Spring. The end of the line is Union Station Washington DC. You could also drive from this CG to Frederick Monocacy Station and pickup the Commuter Buses to Metro or Marc Trains there. There are also Commuter Buses that Pickup in Hagerstown MD, there are 2 CG's up in that area. You also have several CG's in Gettysburg that you could use, The drive from there to the Frederick Monocacy Station is about 30 minutes about 35 mile drive down RT 15.
Hope this helps.