You have to find the shut-off valve, which should be fairly close to the drain pipe from which the water is coming. Look under beds/sofas, inside cabinets and vanities, etc. etc.. [edit see below -- roadside exterior rear storage compartment -- look on the right inside this compartment for a drain valve, and also under the dinette roadside seat for two other valves.]
Do you know that the plumbing worked correctly before you got the trailer? If it was not winterized properly, the low-point drain may actually be closed already, and you have a busted pipe from frost damage.
Do you have an owners manual with the trailer?
Good luck!
Peter
PS -- I think this is your manual:
https://www.airstream.com/wp-content...ibambi2001.pdf
On p. F-15 [PDF p. 75/135] is a sketch of the plumbing system for the 27' SB Safari. Which of the drains is the one in question? In the middle of the sketch, it refers to 2 low point drains typical ["Typ" = arch. drafting term of art] but not clear where these drains are IMO.
On p. F-8 [PDF 68] it talks about the drains as being brass below the trailer, and maybe one more on the right as you look inside the roadside rear storage compartment. Plus two valves under the roadside dinette seat. The old owners manuals were not necessarily accurate for each model, but this one gives you a variety of places to look. Good luck!