If the promotion is something you want and includes job tasks that you can do, go for it!

You are under no obligation legally to notify your employer about any health condition; however, if your condition worsens in the future to the point where you're having difficulty doing one or more job tasks of whatever position you're holding at the time, notify them at that time and ask for a "reasonable accommodation" to help you do your job tasks better. This is according to the federal americans with disabilities act. Depending on what problems you might develop in the future, you can get with your doctor to determine what sort of accommodation might be the one you need in your particular case. Just write your explanation and request up in a letter to your employer, include a doctor's statement which verifies that you have this disease and this particular inability to do said job task, and ask for the accommodation.

Your employer will have to at least consider your request and if it is deemed "reasonable" they will have to comply with it. However, they are the ones who decide whether it is reasonable!

But in the meantime, hey--do what you can for yourself, including going for that promotion!

If it is what you want, go for it. As long as you think you can handle it. Stress can agrevate your symptoms. If they are understanding people and they have enough info on ms, they may work with you. Why don't you sound them out with a hypothetical question and see where it goes from there. Good luck.