According to my notes, the gray water tank was an option on some model Argosys in 1972 but was not standard equipment so your coach may be one that did not receive that option. If it happens to be like the Minuet 6.0 Metres (1977 to 1979), even with the gray tank not all of the gray water is collected - - on the 6.0 Metre Minuets, the shower drain is not served by the gray tank.
Good luck with your investigation!
Kevin D. Allen
WBCCI (Lifetime Member)/VAC/Free Wheelers #6359
1964 Overlander International/1999 GMC K2500 Suburban (7400 VORTEC/4.11 Differentials)
1978 Argosy Minuet 6.0 Metre/1975 Cadillac Eldorado Convertible (8.2 Liter V8/2.70 Final Drive)