I have battled with this, too. One prescription that works is clindimyicin. Its liquid, does not have to be refridgerated, and is easy to use. While the mouth is inflamed, some type of anti-inflamtory or pain medication helps. I use either Turbogesic (vitamin/pain med from vet), or, in a pinch a baby aspirin. My vet actually recommended using the baby aspirin. (NEVER Tylenol.)
I feed a no grain/high protein dry food that they love, and if necessary, I put some in a small bowl and cover with hot water. Once its soft, some cats prefer this.
I also feed various wet foods (wellness, avo-derm, etc.), and located a canned mackaral by Evangers, that comes with a nice gravy that the cats love. Those with sore mouths will clean it right up. (The food smells a bit like sardines, which attracts many cats, and is better for them than giving them human tuna.)
Last, I, too, believe in raw meats. I can't feed these exclusively, but work them in. I do raw chicken livers a few times a month. They come in a carton with a lot of "juice" and this can be lapped up or syringe fed if necessary. The iron and B's in it can really perk up a cat. (Don't feed it every day, though.)
My cats like small pieces of raw pork (which despite what people think, is safe to feed now that the pigs are inspected by the government), and small pieces of raw chicken and raw turkey.
I don't add any more fish to their diets (since you never know about mercury content), and I never feed beef. This is because their dietary systems are not set up to process beef (they would never take down an cow in the wild!)
If the raw meat is in a "rough grind", the cats with sore mouths can still eat it, usually.
Try lysine, too, an amino acid that helps if you are experiencing a virus of some type. Comes in various forms, like gel, powder, liquid, etc. and its good for them.
There are oral rinses available through your vet, but I have yet to find a cat that will let me use one.
P.S. I run a large cat rescue, so I have had more eperience with this than I would like.