this is terrible. Have you persued all avenues of litigation. Someone failed in the inspection to have not found this. First of all, I would keep all records, document expenses, take pictures, and make copies of all said and done. You should be able to do something legal about this to assist you some.
You know as well as I do that you need allot of ventilation under neath. You need dry air and sunlight. Don't ask me how. Bleach will kill mold and it wil not hurt the undercarriage.
Sorry, I just cannot give you any assistance. Let us know what else occurs.