Dry mouth is termed xerostomia. The causes can be;
1) high body temperature due to hormonal disorder like hyperthyroidism,
2) dehydration or inadequate drinking of fluids ,
3) anticholinergic drugs,
4) Mikulicz disease (leukemia infiltrates in salivary glands),
5) Sjögren syndrome; when salivary gland are affected by autoimmune disease (ruled out in your case),
6) history of radiation which can destroy the salivary gland (just to name the possibility, may not be relevant in your case)
7) antidepressants and other drugs
8) Low thyroid levels
Following measures can be helpful;
1) Eating easily ingested foods: Moisture in mashed potatoes makes them easier to eat than potato chips, which are drier. Corn from the cob contains more moisture than dry popcorn. Crackers and biscuits, may be avoided as they are too dry to eat
2) Adopting strict oral hygiene regimens, including flossing, careful and thorough brushing, and the use of a periodontal cleansing aid. Frequent dental appointments for topical fluoride applications and pit and fissure sealants.
3) To stimulate flow using sugarless chewing gum or candy.
4) Oral pilocarpine is also effective in increasing salivary flow.
5) Overall body hydration through ingestion of adequate water every day is very helpful measure.
6) Saliva substitutes (can get without prescription, as they are water-based, they help).
7) Avoid habits, such as smoking, mouth-breathing and intake of caffeine-containing beverages (if you do).
8) Avoid medications like chlorpheniramine, doxylamine, dexbrompheniramine, triprolidine, brompheniramine, diphenhydramine, pyrilamine and phenindamine.
Please feel free for your follow up questions.