Welcome McGill and congratulation on you first Post.
If you are referring to the grey water tank (sink and shower waste water), Airstream did not put these tanks in until 1973. You will either have to purchase a portable tank. These tanks are available in sizes starting at 5 gallons on up to 32 gallons.
If you are staying in a proper RV park, you just hook up to the sewage system and drain away.
-"You want to make it two inches - or, if you're working in centimeters, make sure it's enough centimeters for two inches."-Red Green