Acid reflux could cause sore throat. Simple dehydration can make the tongue feel dry and "coated".
Taking antibiotics or steroids can cause thrush, even sometime after you've taken them.
Maybe a tongue scraper would work? I've seen them at the drug store, some toothbrushes are made with a scraper right on the back. It's suppose to help keep the tongue clean. I think it's a Colgate 360 brush.