Hello @ajbrown22. That is quite a frustrating situation and so far you have had a good deal of diagnostics. Not finding anything wrong is often a good thing with serious medical issues, but also provides no answers. Has anyone done bloodwork and looked for an autoimmune cause? I would be a bit suspicious of the water fasting.....why do you do this? Also timing of the tattoo is questionable. Is this your first tattoo? An allergic reaction to ink in some way? Infection transferred via tattoo needle? Have the doctors suggested trying an antihistamine to reduce secretions? Both diphenhydramine (Benadryl) and chlorpheniramine ( Chlortrimeton) can cause a dry mouth. I even get dry mouth from Claritin. Would daily dosing help your symptoms while pursuing answers? I would continue to drink more water than usual as a dry mouth stimulates saliva production. Gum chewing if your gum contains Xylitol also stimulates saliva production. Check your routine medications and foods for Xylitol as it is a common sweetener and is used in medications meant to increase salivation like Xylimelts. I have to use it nightly for dry mouth since my cancer surgery removed Parotid salivary gland and radiation fried my mouth.
So there is my veterinary viewpoint, take it for what it is worth. Let us know results of your CT please.