this thread might help you guys. Don't use febreeze or fresheners - you're just breathing in piney-smelling mold and nastiness, which has lots of fun long term health effects. You need to kill what's in there, with bleach or another anti-fungal.
https://forum.ih8mud.com/60-series-wagons/226026-how-can-hvac-system-cleaned.html
https://forum.ih8mud.com/60-series-wagons/226026-how-can-hvac-system-cleaned.html