I have noticed from my personal experience that CPAP causes me to have an extra dry mouth. It really does not matter what type of mask I use or what humidity setting I have, I tend to always have a dry mouth. I think just the nature of CPAP in that air is blowing into your body causes dehydration and dryness, especially in the mouth. I use Therabreath mouthwash and it seems to help with the dry mouth quite a bit, I would recommend it.
Joel