Running new duct under home is done often, it allows for better duct as mh ductwork is dismal at best.
It ends need for mh duct rated motors, gives balanced airflow, etc., etc.
IF cold temps are not a problem, easier done. IF cold temps are a problem, have to make arrangement for keeping water pipes from freezing.
That is definately mold as any time your drain pan overflows, it will go in the duct.
Best time to do the duct change is when replacing the furnace , then you can go with furnace to meet duct specs.
Still have to have a mh rated venting/combustion air for furnace if gas though.
Will need an hvac tech with mh experience to come out and consult on it.
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