I bought a new one for my '74
Sovereign from Inland RV. Google them and either call or email them, they'll probably still have them in stock.
However:
Check
very carefully to see if you have the famous "tail sag" (or frame sag), especially if you have the rear bath model.
That COULD be why your original tank cracked, due to increased pressure on the inlet, which is of course where the toilet sits.
__________________
“Courage is being scared to death, but saddling up anyway.”
...John Wayne...........................