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Myasthenia gravis is caused by a defect in the transmission of nerve impulses to muscles. It occurs when normal communication between the nerve and muscle is interrupted at the neuromuscular junction—the place where nerve cells connect with the muscles they control. Normally when impulses travel down the nerve, the nerve endings release a neurotransmitter substance called acetylcholine. Acetylcholine travels from the neuromuscular junction and binds to acetylcholine receptors which are activated and generate a muscle contraction.

In myasthenia gravis, antibodies block, alter, or destroy the receptors for acetylcholine at the neuromuscular junction, which prevents the muscle contraction from occurring. These antibodies are produced by the body's own immune system. Myasthenia gravis is an autoimmune disease because the immune system—which normally protects the body from foreign organisms—mistakenly attacks itself. Read more > http://www.ninds.nih.gov/disorders/myasthenia_gravis/detail_myasthenia_gravis.htm#222033153

So, in a nutshell, drinking does not cause Myasthenia Gravis. Drinking alcohol causes weakness and since Myasthenia Gravis is characterized as a fluctuating muscle weakness, drinking alcohol would increase the chances of weakness and fatigue. 

Read here for a good write up concerning drinking and Parkinson's disease > http://answers.yahoo.com/question/index?qid=20100606151411AAYWENV