You have two brass valves to drain the hot and cold water lines. Mine are both with a red cap over the lever... where at least one red is hot. Drain those when you reduce the water pressure with a faucet inside to make it easier to drain.
I would suggest Airstream put a BLUE plastic over the cold water valve. Duhhhh.
The cheap, poorly protected white plastic Piece of Carp is to drain the fresh water tank as Thurston III indicated in post #6. It can be stubborn to move and can BREAK. Be careful. You get some calcium carbonate from hard water in behind and it acts as a grit and you can even break the lever off. (This can be temporarily or permanently repaired using a small cork to plug the drain and a wire strung between the two screws to secure it.)
It was obviously an Engineering Breakthrough of inconvenience how the draining system is hidden from view and easy access.
Someday Airstream may replace this plastic piece with brass. Even gravel while traveling can break this valve off flush. Thus the CORK to plug the DAM idea. Like the Dutch Boy and the dike in Holland. We had ours replaced at Jackson Center with our 23 foot petcock being knocked off flush. I asked they put a piece of bent tin to protect it, which they did... and it worked.
The bent tin below is standard. Just spend another ten cents and secure one with tape or cement in front with the same idea. Protect the Goods. Like a CUP if you play sports, kind of thing.
My last two Airstreams I added the tin to protect this plastic petcock. Works. Otherwise... you may be in a "Real Pinch without a Cork".
I love the 23 foot today. It now has 15 inch wheels, rather than the 14 inch stuck with Load Range C tires. Goes anywhere. Tight parking... no problem. Even 16 inch Michelins and Sendels can fit if the need arises. Enjoy. We had our for many years.