Have you tried mouth tape?

Every so often I charge up by setting myself a 30 minute timer to drink a bit of water. After like 3 hours my hydration levels are high enough that my mouth waters and my alertness peaks.

Hard to balance at night, but a humidifier using distilled water near the sleeping area will slow the rate of drying.

you can pack your mouth with cotton balls before going to sleep! very effective at producing a dry mouth.