a mouth as dry as . . . . . . . . . . .
it's a common problem when you use CPAP
it's caused by an open mouth when you sleep
 . . . muscles relax and the extra air wants to find a way out.
it's like a windy day, the washing dries very quickly due to the extra air taking out the moisture. 
        The same thing happens inside your mouth.

Is this happening to you?

If so, Plan A is to try a chin strap.
Why? Because the pressure exerted on your chin will often keep your mouth closed, and the extra air then has to go out the mask exhaust, just like it is supposed to.

For most people a chin strap stops the escaping air, or dramatically reduces it. 
That means little or no dry mouth or the feeling of air bubbling through your lips as you go to sleep.

Chin straps can also sometimes help prevent air swallowing (aerophagia) and the discomfort it causes.

A warning though, the higher your cpap pressure, the less likely that the chin strap will do the trick. 
Consider moving to a full face mask. One that covers your nose and mouth. 
That way the air has no where to go but where it is supposed to. 
And the wider "wheelbase" of a full face mask makes it far more stable than smaller masks under higher pressures.