Definitely sounds like post nasal drainage. I always recommend trying a nasal wash like Xlear, which is all natural and does not cause dependence. Also do your best to remove any and all alergens from the sleeping area. That means stuffed animals, carpets, draperies (you want natural cloth that is untreated), and make sure that there is no mildew on the pillows or mattress. Mold and mildew in the walls may be problematic as well. If you can improve the air quality and give her immune support at the same time, you will really make some progress. (Remember exhaled waste is accumulative, so leaving a window cracked may also be helpful)
Many blessings.
If the endoscopy, including examination of your daughter’s stomach, was clear, it is highly unlikely that her upper intestinal tract is the source of the mucous. A rarely occurring exception, in children and adults, is a condition called Eosinophilic Esophagitis, in which the esophagus can appear grossly normal on endoscopy. This condition is usually accompanied by swallowing difficulty and not just vomiting.
The most likely diagnosis is that she has excessive post-nasal drip, from either her nose (especially with nasal polyps) or sinuses, especially at night and that she is swallowing this mucous all night long.
Good luck