Curly in some areas you can self refer your child to the ED Camhs service, normal Camhs will be no use to you. Who offered your dd the sertraline if not the ED service?
Insist on bloods, ED can massively mess up the electrolytes which can be fatal (low potassium for example). Is your dd going to school? If she isn't eating she should be at home on sofa/bed rest. No sport at all as her heart may already be struggling.
Fwiw my dd was seriously ill at 90% wfh, her heart rate was in her boots, low bp, dizziness, cold all the time etc. I was in denial about all this until it was spelt out to me how dangerously ill she was 😕
Get yourself a copy of the Eva Musby book, its brilliant esp for younger kids like your dd.
My other thought is have you had a really good look through your dds phone? Take away any social media, check for any fitness/calorie counting type apps and delete them, put restrictions on the phone for anything like calorie counting or ways to lose weight etc.
How much has your dd eaten over the last 48 hours? Less than 500 cals a day requires urgent medical attention.
Can you get signed off work for a while? It's absolutely impossible to care for a child in the early stages of FBT and work imo, it requires all your strength and time 24/7.