Shari, I agree totally with Cindy! I was diagnosed in the mid 70's by an MG specialist, but in recent years have seen 2 neuros who wanted to take away my diagnosis because I am sero-negative, and EMG negative.
All of the symptoms you mentioned are most definitely MG symptoms. My hubby could always tell when I was especially fatigued because of my "half-mast eyes", as he called them. When my neck muscles fatigue, I feel like I need to use my hand to hold my head up. I also choke when fatigued, but in my case it is more likely to be on my own spit first, then possibly on a liquid. I very rarely choke on solids. If I do choke when eating, it is usually the spit that goes down before the bite that causes the problem.
If you can locate an MG specialist, you will be better off. Most neuros just don't know much about MG. And having a SFEMG is a good idea, but even it is not 100% positive. I have read of MGers that are negative in everything, but were diagnosed on symptoms and their improvement on Mestinon.
And as far as the negative chest Xray goes, not all MGers have an enlarged thymus. At least, I assume that was why you were given the Xray. I googled MG and thymus and gleaned the following info: "Computed tomography (CT) or magnetic resonance imaging (MRI). These tests can help identify an abnormal thymus gland or a thymus gland tumor." If it takes a CT or MRI to identify an abnormal thymus, I'm not sure what good an Xray would do.
Unfortunately, knowing you have all the symptoms of MG may not do you much good if you are stuck with this doctor and he is convinced you don't have it. Sometimes if you start looking for another doctor, they figure you are "doctor shopping", looking for one who will give you the diagnosis you want.