before you write us all off...
try what teddy bear suggests:
Operate you fan "on" mode for a couple days to test "the condensation in the ducts" theroy.

personally...I'm not sure about that, but would be interested in what you find out. as teddy bear
says..its a learning experience.
better to resolve what is happening than to live with it...right?
investing a week of your time may provide a solution to your RH issues.

living in the south...I've seen the issues caused by the whole chinese drywall fiasco.
corrosion of copper in electrical, hvac causing homes to be re-wired, hvac system completly
changed out...breathing issues. its nothing to make light of.

keeping the thread on track is always difficult. so just ignore what doesn't apply & use
could help you. insulting homeowners is not the intent of this forum.

best of luck