I had low vitamin D, and my doctor told me that might be the reason I was feeling so tired. I was doubtful about this comment, because I knew I'd been exhausted even at times when my Vitamin D levels were fine.
I was also told Vitamin D is more common in people with Crohn's.
My doctor prescribed a Vitamin D/calcium supplement (I'm not sure why as my calcium levels were fine). The supplement seemed to upset my stomach though, so I decided to try and get Vitamin D naturally, from sunlight. I used to always put sun cream on whenever I went outside because I'm so pale I burn really easily, even here in England where there's not all that much sun! But I tried only putting sun cream on in the afternoons when the sun is brightest, and leaving it off in the mornings, even when I went outside.
A few months later I had the blood tests again, and my Vitamin D was normal. But I was just as tired as ever!
I think it is true that lacking Vit. D can cause fatigue, but us people with Crohn's can have many factors which make us tired. Since it's easily fixed, it can't do you much harm to get some more sunlight (as long as you don't burn) or try a supplement, and you might be lucky enough to find that stops you feeling tired.
I know it might not be possible depending on your schedule, but the afternoons is quite often when my energy peters out, and having a nap for an hour or so makes me feel so much better!
Another possible cause may be that you're not getting in enough calories, if you're either not absorbing properly or if you don't have an appetite. If you're losing weight try having some extra snacks, high calorie drinks or supplements to keep you going.
I hope you feel better soon. Healthy people don't tend to appreciate just how debilitating fatigue can be. Telling people you're feeling tired sounds like no big deal, but I know how hard it is to function when all you want to do is sleep!