You got a lot of great info here…I have fibromyalgia as well; that form of fatigue felt like I couldn't hit the couch fast enough…but the MS fatigue make me feel as if I am slogging through thick molasses. I also have a narcolepsy-type of fatigue like Erika described that makes me suddenly fall asleep, which scares me when I'm driving. It occurs during mealtimes too.

I agree that you shouldn't use terminology but rather what things feel like to you. For instance, "Sometimes I feel as if I am walking through wet concrete," rather than "I experience fatigue." It is the description of the symptoms that helps the docs understand.
When I saw my neuro a couple of weeks ago with severe spasms in my legs, he asked me, "Does it feel like stinging?" I said, "No, it feels like a mop being squeezed." He totally understood what I was experiencing.