What I'm going to tell you will not give you comfort. My cat, at 13 years old, did the same thing. She had a brain tumor, or that was the final diagnosis. She had a head tremor and she would shake her foot like something was on it. She also stopped eating, and they were also checking her kidneys. Her problem ended up being neurological. In the end she was wobbly, and was acting very strange. She would stare vacantly for hours. At the very end, she was hiding a lot. A kitty hiding is a very sick cat indeed.
They gave her steroids, and she was better for about one week, and then went downhill very fast after that. I couldn't end it, and I came home on the last day and she was in a coma. One of the worst days of my life.
You need to keep plugging away with the vet. I hope this is not the same condition as my cat. I totally understand what you are going through, I loved her so much, and you feel so helpless. I think it stinks that we can outlive so many pets. They should at least have a life-span the same as ours.