Hi, I just read your post and I feel so bad for you and your family. When I was a kid I was also sick all the time and nobody except my mother and grandparents(and sometimes my father!) believed me. Everyone thought it was all in my head. They even had me see a child psychiatrist who told them basically that I was truly sick and that they were the ones with the mental problems. (although I did keep going to her anyway because it helped me to be able to talk about how being sick all the time made me feel). My pediatrician was absolutely wonderful. He knew I was sick too, (it turns out that I have severe allergies) but he did not always know how to treat me. None of the other doctors that he sent me to believed that I was sick. But my pediatrician was a real angel. Finally, my mom found an pediatric allergist to bring me to, and he did extensive tests and was able to pinpoint the cause of my illness. I know it sounds like it's not that bad, but allergies can really wreak havoc on your system. There wasn't a week that I didn't have an ear infection. But this doctor was able to help me a lot. I still go to him.
I now have severe eye problems. I have been told by a few people that maybe it is in my head. I brought that up to my eye doctor, and I said that I am starting to believe it myself. You know what he told me? He said "If it were in your head, how would I be able to see the physical symptoms?" This is what you have to keep telling yourselves.
You know your daughter better than any doctor does. If her symptoms are this serious, there is definitely something wrong. Has she been tested for something like chronic fatigue syndrome or Epstein Barr Virus? (that can cause extreme fatigue, soar throats, fevers, etc...)
Good luck, and I hope you find some answers. And when you do, go back to that first doctor and throw the results in her face. I just hate it when doctors think that they know everything.