I'm sorry, I know I have read a lot of your posts already but I can't remember your details. Did you have a brain and spinal MRI? Your descriptions of heat intolerance, muscle cramps, and fatigue do seem classically MS in my untrained opinion. I went undiagnosed for four years because my brain MRI's were clear. This February I was finally checked for spinal lesions and subsequently diagnosed with MS.
I hope you get things figured out soon. Be persistant!
Be joyful in hope, patient in affliction, faithful in prayer (Romans 12:12).