Try mobile home parks, call to see if they accept RVs in addition to permanent manufactured housing. This will depend to some extent on housing codes that vary locally.
You might have to expand your idea of what is the "DC Area." With respect to residential areas for people who commute daily to the Capitol, DC area now encompasses most of Maryland west of the Chesapeake (including Baltimore) and in Virginia as far as Winchester to the west, Fredericksburg to the south.
The further you get from the District itself, the more likely that property values are low enough for either RV parks or trailer parks. College Park is so costly because it is so close.