It sounds to me that you may have a leaky roof. This is exactly what water leaks look like in drywall. Any idea on that?
Kilz is a very good product, but since its conception, there have been new and better products out. I really like Zinser B-I-N. Its like Kilz but much better. In my opinion from what you are stating, you have a small leak and its leaking at the very same spot each time. The Kilz should have taken care of the stain, but if it is ongoing, it will show up again and again.
The mildew in the bathroom can be killed by spraying a mixture of bleach and water on the area. you really need an exhaust fan to carry out that condensation. your paint peeling is also due to not preping the area properly and again water being on the paint. You may even be putting oil paint on latex paint.
You need to fix the water leaks and vapor before you do anything else. you are just wasting your time now.