Living in an Airstream in DC during the winter though romantic, would be a little tough. Take tonight, temps are going to be in the single digits and keeping the water lines from freezing would be hard to do. Even getting around that, you're going to have to find a place to park or have an outside/inside storage place that would allow you to live there. Your NW DC area (which is nice) you might get lucky and find someone that would allow you to park it in the back yard using their elec, water, sewer connection, same would hold true in NE or SE DC which even though the areas are getting better, could be a bad place to live in a trailer unless you have someway to secure it with bars on the windows (which is a look I have not seen on an Airstream)
Have you ever thought about a houseboat in DC? You might have better luck down on the water.
If nothing else, you could always live in a van down by the river! ;-)