Bob,
My 1976 owner's manual lists the following tank capacities:
Main holding tank (Black) (Int'l Rear Bath) 20 gal.
(Center Bath ) 23 gal.
Aux Holding tank (Grey) (Int'l Rear Bath) 14 gal.
(Center Bath) 16 gal.
Fresh Water Tank 50 gal.
Water Heater 10 gal.
Hope this helps.
Oliver Filippi
P.S. The Owners's Manual covers all models (lengths) of that model year.