My business is in mobile home repairs and we run into this problem almost daily. I have never had to return to a unsatisfied customer! Unfortunately like glass the crack will travel after time. Drill a hole in the bottom of the tub. Then shoot in an expanding foam sealant in. Use enough to fill the entire bottom cavity and let this cure at least 48 hrs. This will give the tub additional structural reinforcement. Then sand and patch with a fiberglass patching kit, much like a boat this will seal out water. For appearrance sake recoat the tub with tub epoxy paint. Good Luck!!