I absolutely agree with
@Gingergrrl . I realize that few ME/CFS patients go to their disability examiner appointments armed with the amount of supporting medical documentation she has. But, disability can be, and has been, awarded with much less clinical proof. So don't be too concerned if you haven't had significant testing with abnormal results.
If you have a supportive, knowledgeable doctor, and if you are concerned the testing might include activities that would cause harm, your doctor may be willing to provide a written exemption. This happened for me at a re-evaluation assessment, although my doctor took the initiative. He knew the test would involve four consecutive
hours on a treadmill the first day, and three hours the second day -- this despite the fact I had always held a desk job. He wrote a note that I was not to do a treadmill test. Since this was all they had planned for me, after much discussion they sent me home. Perhaps their intent was to kill me!!! Regardless, my benefits were continued.
Another issue in the above situation was "informed consent" -- or rather, "uninformed consent". Before the appointment, I was expected to sign a document agreeing to do all rehabilitation they prescribed based on the results of their evaluation. At this point, I wasn't even aware of the intended "treadmill torture test". I marked up the consent form, indicating I would not, and could not, agree in advance to unspecified rehab; and further, that no rehab could be prescribed without my doctor's prior approval. It seemed no one had previously refused to sign this release form without editing it.
Probably the only advance preparation you can do is carefully consider how your condition impacts your ability to do your job, or any job. Make notes, and review them frequently until they are clear in your mind. Comparing yourself to the functional capacity charts available on various ME/CFS websites to determine where you "fit" may help.
Most of all, tell the truth. That way, you won't slip up if they attempt to discredit your words. I recall reading advice (after the fact) to tell disability insurers as little as possible. I did the opposite. I told them everything, with the hope that a doctor reviewing the details would come up with a different, curable/treatable diagnosis. This didn't happen; but, perhaps my detailed records were difficult to dispute in the absence of conflicting evidence.
I, also, would advise against giving depression, or any other false condition, as the reason for your inability to work. Depression can be treated, ME/CFS not so much. Some disability policies/programs exclude, or limit, coverage for mental illnesses. Regardless, if we want ME/CFS to be taken seriously, we need to acknowledge its reality, and not hide behind other more accepted diagnoses.
@sickntired771 I wish you all the best. Do let us know how things go.