I am not sure "relatively rural" near DC exists - depending on your definition of "near". We lived in Frederick County MD and had 6 acres and was able to keep my MH in my driveway. Very rural where we lived - actually about 2 miles from Camp David in the Mountains. But that was a house, with water, sewer and electric. In MD zoning for keeping an RV isn't state or even County specific. It comes down to covenants in a particular neighborhood. But again, that's residential. The further you get away from DC the more available and affordable land is. Where we lived was about 1.5-2.5 hours away from DC depending on traffic (and there is a LOT of traffic via 270). Lots of people commute to DC from Frederick and even as far away as Washington County MD. I can't speak to things heading East toward the bay.
My advice, pick a distance you can tolerate a commute, contact a realtor, and start looking at land. But buying a plot of land just to keep an RV with infrastructure to support it, would be something akin to a modular/Mobile home. installing Septic or tying into public septic is costly and requires a lot of paperwork and inspections. In some counties putting a hole in the ground will cost you $20k in impact fees.
Maybe a Farmer would lease you a piece of land and avoid the headache altogether. Or look for "game land" or "recreational use" property and get someone to hook up electric and sewer on the down low.
