There are certain key points you have described:
1. All of the tests are normal. These should include a lung function test for sure and possibly a chest x-ray
2. The cough goes away as soon as she falls asleep.
Most causes of cough that come form the lungs (bronchitis and asthma included) actually get worse during the night when someone is sleeping. The mucus pools in the lungs in these and causes the person to wake up. Another source of chronic cough is a sinus infection. This, too, is often worse at night.
I would speak again with your physician about the different possible causes of coughing, including the possibility of a non-organic type of coughing pattern.
I to have had good results from drinking water. I used to drink a gallon, yes a gallon of water early in the evenings while in the military. Sure you visit the bathroom constantly but the benefits out weigh the trips and the trips to the bathroom subside quickly. Before I started doing this I had very bad acne, haven't had it since don't drink water like that anymore, second my asthma in the middle of the night was no longer a problem, though since I stopped drinking that much water it has returned from time to time but I deal with it. I don't recommend a gallon but the benefits from simply drinking water is a proven plus in my book.
Hi, have you heard of the watercure? It's supposed to be very effective at dealing with asthma
1. Drink 8 glasses of water a day.
2. 1/2 teaspoon sea salt a day.
Your daughter may well be dehydrated. Please make sure she drinks enough water everyday - she must cut out all cola and any other drinks which contain caffeine.
http://www.watercure2.org
http://www.watercure.com
Try it, its free! and judging by what I've read about this watercure, everyone with asthma who's tried it has been virtually cured of their condition completely!