If you were a regular employee you could ask for accommodations related to MC such as a warm environment (for instance I was allowed to keep a space heater under my desk), access to elevators rather than having to use stairs, and notification of any pesticide applications with a few days off work if needed.
Rather than just setting up a meeting, I would advise getting a referral to an occupational therapist. They will meet you at your workplace and evaluate what might be helpful to you in your work environment. Since they are healthcare professionals it will have more clout than you trying to explain your condition to the employer or supervisor.
You can also use the services at your state's Department of Vocational Rehabilitation. They were extremely helpful for me when I was looking for jobs that could accommodate my conditions. They also have a list of employers who will hire people with disabilities. You have to go through an orientation, then get an appointment with a counselor, and then they will order any medical evaluations or occupational therapy consults. It's not a fast process, but if you meet the guidelines (I think they have income restrictions on some services) it's well worth getting registered.
If you have performance issues related to pain, fatigue, etc, then it's probably best to get evaluated and find the best fit for your condition. That may be telecommuting so that you can work when you are alert and feeling better. I certainly understand all the problems and have had some really bad bosses in the past. But I have had some great ones, too.
Did you get gene sequencing for the SCN4A mutations, also? They tend to cause more weakness. I have Thomsen's but also one variant in SCN4A which is probably why I'm so much more sensitive to potassium and cold. And of course we're just as susceptible to fibromyalgia and other conditions that cause fatigue, pain and weakness.
This is a good article you might want to share with your supervisor. It's specifically about periodic paralysis, but Thomsen's is an ion channel disorder with very similar symptoms.