One of my cats has had a tilted head since she had a possible stroke as a kitten. I found her one morning rolling on the floor because she had no sense of balance. The vet could find nothing wrong with her and she has been fine for 8 years,just with a little tilt.
I wonder about a head injury. So just the head tilt, no twitching or odd eye movement or stumbling? Has your cat been tested for FLV ? (leukemia) Kitty may have taken a fall and you didn't see in which case a prednisone shot might be helpful with any inflammation if this is the case.
My cat had an ear infection recently and she was on antibiotics for TEN days. Maybe try a different antibiotic for a longer period of time.
This is all I can think of, but at least my answer will bump your question to the top again so maybe someone else has an idea.
We have tried the Baytrl antiboitic in the hopes of an middle or inner ear infection with no response - nothing else seems to be wrong with this kitten. Any more suggestions
Oh, I missed the part where this kitty had a upper respiratory disease so maybe a very deep ear infection that the vets could be missing although 3 vets missing something like that, does seem unlikely.
Charlene, I just googled "kitten and head tilt" and a bunch of sites came up. Several describing the possibility of what it could be which may include a Vestibular problem (middle ear infection,) punctured ear drum, head trauma, idiopathic (no known reason.) Also, sites of people with kittens of their own with head tilts. While you're waiting for someone who might have experience here, you might google this in the mean time if you haven't already.
Best of luck in finding an answer.