Two things pop into my mind?

First is the meds that you mite be on…not only for the neuro pain but for other issues? These can dry out your mouth especially when you take more than one…. I use Biotene oral balance gel… it’s a squirt bottle, has a strange taste that you can get used to but it works to help keep your mouth moist. It’s also great for fresher breath! Go figger. In my case, the dry mouth is meds induced.
The other is that thyroid or Sjogrens immune issues mite be going on concurrently? Something to ask about, read up on them. Sjogrens is an immune issue that dries out the moisture glands in the body. It’s a painful condition for which there is no cure.
I hope this is useful in some way. Let us know.