>> Could some fungus that has no obvious symptoms except for the oral thrush be the source of all my misery?
Absolutely. When the doctors don't know whats wrong, it's likely Candida / auto-intoxication. Your symptoms are both typical candida symptoms. btw, most of us don't get rashes, but everyone reacts differently to auto-intoxication, that's why doctors can't diagnose it easily (that and they don't believe in it, retards).