Since your gray water tank leaked at 3/4 full doesn't mean it has a crack.
Some of the drains going into a gray water tank are not sealed completely. They
simply stick into the tank and are sealed with plumber putty. Some of the
drains are in the middle rear of the tank instead of at the bottom rear. (not a
good idea but Airstream sometimes had a lot of those) You might check that seal
before thinking about replacing the gray water tank. jauman2346@aol.com