Have you actually pulled the unit and found the crack? Reason I'm asking is that it would be a shame to just assume it's cracked and it be a check valve leaking or something simple like that.
If it's cracked and is an Atwood, It has an aluminum tank that can possibly be tig welded. If the crack is beyond repair you can purchase the tank only. I'm not sure of your options if it's a suburban because it has a steel tank with some type of glass coating inside, but surely it can be replaced, assuming the rest of the unit is still in good working condition.