@Littleman -- Although there is not one disease called dry mouth disease, some diseases can cause dry mouth.
For instance, anemia and diabetes often present with dry mouth, and it can be a side effect of Alzheimer's disease as well.
Really, dry mouth can show up with a lot of diseases and conditions -- arthritis, Parkinson's, even stroke and hypertension.
However, dry mouth is not always caused by a disease. It can also be caused by smoking or chewing tobacco, and often shows up as a side effect of many medicines.
If you are having such bad chronic dry mouth, I would ask your dentist about it; he or she will be able to help you out.