I have the same diagnosis.
I have posted the following on several threads on this forum, not to defend the medical providers, but to try to steer people clear of anger and helplessness, which are counter productive to feeling better.
In my experience, and with my knowledge (I am a retired MD), I find most of my own providers are generally helpful and knowledgeable. This is a very frustrating disease (for the patient and the professionals).
My type of PN is idiopathic, which means the cause is unknown, and not for lack of trying. 40% of patients with small fiber PN are idiopathic. This simply reflects the current state of medical knowledge about this disease. It is generally not because of ignorance or lack of effort. There certainly is a cause; it's just that that cause or causes have not been discovered yet. It will be in the future. Who knows when?
I believe a comprehensive list of tests has been published by Dr. Oaklander of Mass General. Perhaps you will feel more confident if you download the list and discuss it with your neurologist or PCP.