Not every metro lot has room for a 22' motor home. With that, do not use College Park Metro station, there is not enough parking. Your best bet is Greenbelt metro station which is directly off I95 with its own exit and usually has space for an RV. Many of the other Metro parking lots have limited ground level parking and parking garages now. At least it was 2 years ago, though may be full now.
There is another option, there is a train line that runs all day from MD to union station. It is called the MARC Penn Line trains. There are several stations that have large free parking such as Bowie State college that you can catch the train from. But it is a commuter train and only runs until about 9pm, however, Bowie state is not that hard to get to and has a huge lot on the far side of the tracks from the college. The other MARC lines do not run into the city other then during rush hour.