Can't ignore this
My brother even often worked outside, it's not possible he didn't get enough sunlight.

Even though the vit D your body makes from the sunlight doesn't harm you, does not mean supplements won't. Mind, I'm not saying it will, but because vit D is a fat soluble vitamin, you don't just pee out what is too much. A wrong dosage may be harmful in people with kidney diseases, which can exist before noticing any symptoms. So, better check and talk with your doc, he/she may or may not advise you to take vit D, but that's up to the doc. He/she will also tell you your best dosages, which may be 6000 IU or something else.

What did you mean by getting sick in the sun? Because MG tends to worsen when you're warm, so if it's very obvious (15 minutes sunbathing means sudden weakness tha wasn't there before), that may be something to write down...
I get Lupus rash in the sun and extra weakness. Though I have no idea what happens now, because I avoid it because reasons above and Imuran and stuff.

Good luck at your appointment, hope it's soon?