Your's is probably a 45-50 gallon tank. On my Argosy the tank was a 29 gallon tank and you can buy replacements for it. However they aren't near the quality of the OEM units and don't have any baffles inside. I ended up not using the one I bought and had a custom 50 gallon tank made instead.
Check with Mill Supply
for a tank. They would be about the only source I can think of if one exists.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)