Where are the tendonitis symptoms located?
Arms , legs both equally?

Can you list what has been firmly ruled out so far?
I'm assuming they did a MRI or some kind of imaging to rule in/out anything that way.

If you have been in a Lyme area I would make sure to get the best testing to check that out.

If you already went to many MDs and got no help, I would tend to look for a very experienced, highly rated naturopathic doctor and give them a try.