If your skin biopsy for neuropathy comes back positive for neuropathy, consider autonomic testing at the closest autonomic center to your home. There are not a lot of them and not every state has them.

Also, testing for autoimmune disease, such as having an ANA and ENA to rule out diseases like Lupus, Sjogrens etc.

If you haven't been tested for Lyme disease, that too.

That's a start.