[MRM]

75% humidity is very, very high. That's what's causing the through-house mildew smell. You can't do anything about the smell until you lower the interior humidity. Use the fans in the bathrooms and kitchen whenever you cook or shower. Keep them running long after the shower or other use.

That level of humidity will require a lot of dehumidification. I would get two big units and run them 24 hours a day. You'll see a big difference. That level of humidity can't even be comfortable to be in.