I'm so sorry to hear this .
I can't relate how stressful is for you and treatening for her life as I went through the same with Koxka.
Have you tried to give her "human food" like boiled chicken, turkey,etc...
Mines go nuts for raw Porck kidneys and salmon it worth a try.

Antihistamics have also a sedative effect don't think is good if she is already appatic. There are drugs out there to stimulate appetite, your vet should know.