Well, a rheumatologist is the doctor that often evaluates musculoskeletal medical conditions, and other diseases which can affect widespread muscles.
A neurologist often evaluates and treats the same types of conditions.
For pain issues, a physiatrist who specializes in pain management can be helpful with chronic muscle pain issues.
Gastroparesis would be best evaluated by a gastroenterologist.
I am looking for a diagnosis - over 30 years of gradually increasing pain, weakness, gastroparesis, chronic pseudo intestinal obstruction, paralyzed right breathing diaphragm, etc. I have seen neurologists, rheumatologists, and gastroenterologists, and I am on serious pain meds - but no treatment or diagnosis of the CAUSE. They do tests, and say - "Nope, not caused by something in my specialty." Who should I see now? Is there a specialty for muscle disorders?
Unfortunately, there is not a specific specialty which treats "muscle disorders". This falls into the realm of multiple other specialties, mostly rheumatologists and neurologists.
There may not be a single condition which encompasses your symptoms, which may make it difficult for these doctors to find one clear reason for the "cause".
If you are interested in pursuing a further diagnosis, I would try getting another opinion from a rheumatology clinic which is associated with a large medical center -- where they would have a team of people that see the more challenging cases.