Flossie. I forgot to ask if you had a bathroom vent fan? You mentioned your house was old and they frequently didn't have them, at least here.
I have a new one which exhausts at a low rate 24/7, and runs at a higher fan speed when the motion detector sees motion in the room.
If you don't have one, perhaps you could have one installed which must vent to the outside of the house.
Yes, I believe you're going to solve this. Thanks to Mattk3, I think I'm heading in a good direction also. Wish us both success. I started to say luck, but it's far from that. It's a lot of hard work and a whole lot of money.