Dried mouth, back pain and dry eyes. What could this indicate?

Medications? Any medication with "anti-cholinergic" activity could create this combination of dryness, though I think the back pain may be unrelated. Antihistamines have some anti-cholinergic activity too.
See eye doc. See your primary doc and make sure you don't have a viral syndrome or autoimmune problem where your immune system is attacking yourself. Then see eye doctor to llok for sjogrens syndrome.