The toilet tank is traditionally called the "black tank" and the wash water tank is called the gray tank. If your Airstream was built in 1973 it did not come from the factory with a gray tank... well there may have been a couple of overlap units built. 1974 was the first year of regular production with a gray tank and it was only 10 gallons.
Replacement tanks for the older models are usually not available and must be repaired (if possible) or a replacement tank can be custom built at considerable expense. Used tanks can sometimes be found but they are also old and the plastic tends to crack with age. There are universal replacements available in a limited range of sizes and shapes. Do a web search. Good luck.
2000 28' Classic TT
1994 30' Land Yacht M/H