It is scary, isn't it? Our son, who had never even had a sleepover other than with a grandparent, went to summer camp for three weeks when he was 10. It was several states from where we live.
He was fine. And, more amazingly, so was I. I was jittery for a few days, but it was much easier than I expected. He went to camp for four weeks last summer (age 12), also quite far away, and he loved it.
So if you find a camp you trust and your daughter is excited about it, you'll do fine. And if she's hesitant, there's always next summer. Good luck!