Severe mono exhaustion, no other symtoms

My 13 year old continues to sleep 15 to 20 hours per day. She tries to go to school for half days, but on Friday, she went to school for a half day came home at noon, and with only a half hour break to eat something about 6PM, she slept for a full 24 hours. In retrospect, she had been coming down with this for a long time (first visible sign was hives), but she really hit the wall on December 10th and has been sick ever since.

We tried an endocrinologist, but after a full battery of blood tests, she has no glandular problems.

As I've posted before, other than exhaustion and hives, she has not had a single other symptom of mono. No swollen glands, fever, sore throat, enlarged spleen, liver function problems, NOTHING, -- not at the beginning and now now. She had elevated EBV antibodies (which almost everyone has, including 13 year olds) and that's it.

She is not faking this. She desperately wants to get back to school, her life, her sports, and her friends. We are all so frustrated. Has anyone experienced this kind of long lasting severe fatigue with no other common symptoms?