Bob answered the black tank flush question, careful to never mix them up when hooking up water line.
I cut off the plug that never stays put and got a screw-in cap for it. The 2013 and on Airstreams come with a screw-in cap, you can get one at a dealer maybe, or Camping World, Home depot but they may be white.
Doug and Cheryl
2012 FC RB, Michelin 16, ProPride 1400
2016 Ram 1500 Laramie Crew Cab 4X4 Ecodiesel 3.92 axles
The Truth is More Important Than the Facts