I like "debilitating fatigue" too. When someone so "oh, I know how you feel", I usually just ask "have you ever been too tired to chew gum? Because that's how I feel".

Most people I know can visibly see what the heat does to me because I tend to panic when overheated. I don't really know how to explain it to a non-MSer though. Besides just saying "I can't tolerate heat".