Smaller travel trailers like the Airstream Basecamp 16 and the 16RB Bambi and Caravel have a combination black/gray tank.
https://www.airstream.com/blog/under...20water%20tank.
From Basecamp Owners Manual:
Black/Gray Water Tank and Draining
To empty the combination tank, attach the sewer hose
by pressing the bayonet fitting onto the dump valve
outlet and rotating clockwise until it feels solid and
secure