Camping Overnight at Airport Parking Lot

Sure....why not.

Another possibility is, near an urban area, outlying commuter rail stations have large parking lots. Put the $5 fee in the appropriate slot for your space and rest awhile.

In my previous vehicle, a VW bus, I spent many a night in airport parking, paying around $8. I did not pop my top or do anything to bring attention to my situation.
Shall now try the same in the PleasureWay.

At Greater Cincinnati Airport, they have a large parking lot, well away from the terminals, and shuttle busses that drive around picking up and dropping off passengers 24/7. Its also patrolled by airport police, has very bright lights, and is directly below the approach end of the main runway. There aren't any homes, grocery stores or restaurants within several miles, and its expensive because its the only game in town. Did I mention that it isn't even in Ohio? 15 miles from downtown Cincinnati.
The point being, that airport parking lots are designed for empty motor vehicles, not people. I certainly wouldn't plan a vacation around them or expect to get much sleep.

About 3 years ago my husband and I stayed at Chicago's O'hare airport in the Long term parking area. It cost $13/day and we had public transportation into the city. We stayed 2 nights, just basically sleeping in our B rig and touring during the day. No one bothered us and we were too tired from walking all day to notice any noise at night.

I agree about camping versus sleeping...you'd have to avoid setting out your chairs and running the gen! People asleep in cars at airport lots might be more common than we think, if they return late and have a long drive ahead.

This is slightly off topic, but...While thinking about this idea, I googled Raleigh airport parking to see what current prices are. Turns out there is an online company that offers many cities where you can park at airport motels and use their shuttle for $3 a day, no motel stay required.

Each motel decides if they can handle oversize vehicles like vans, and some do at RDU. Might try that next time. Then if return flight is late, We could sleep in the van and drive home in the morning.

I stayed at a small airport parking lot in Redding California.
Truck Camper - fit in a parking space.

Parking lot is locked at around 10 pm and opened around 6 am
Free.

I've stayed overnight in airport parking lots, and have tried to get some sleep. However, I have not camped in one. Setting up a camp is likely to get attention you don't want.

Have stayed at two airports (Houston & Calgary) but I stayed in the cell phone parking area. Parking was free and people coming and going, some waiting a few hours etc. Got asked about 3 am in Houston, and said they missed plane and would be about 4-5 hours late. No problem have a good rest.

For $16 a night, one could stay in Greenbelt Park which is 2.5 miles to the closest metro station. They have water and a dump station along with bathrooms and piece and quiet.

I'm completely for stealth camping but, if I were doing it in DC, it would be downtown, Woodley Park, or Tenleytown, not out at DCA.