OK, thank you for the additional information. Any fever that can't be explained is called a fever of unknown origin (FUO). Most commonly. it is going to be due to some unknown infection. Other causes include inflammation (secondary to a blunt trauma such as falling off of a tall piece of furniture, or being hit by a car, or even a condition called pancreatitis in which the pancreas get really inflamed). Other causes of fever include different cancers, and auto-immune diseases.
It sounds like you and your veterinarian have done everything right so far. Hospitalization for IV fluids, blood testing and x-rays are often the first steps. If nothing is found on these tests, an abdominal ultrasound may be the next step to find out why she is experiencing this fever. I would discuss with your veterinarian her previous blood testing to make sure she has been tested for FeLV and FIV, pancreatitis, as well as had her urine tested to make sure no urinary tract infection is present. For this, a urine culture and sensitivity test is best. If these have all been done, as I said the abdominal ultrasound test may be next.
If this were my pet, I would continue her on the fluids, and start an injectable broad-spectrum antibiotic in case some hidden infection is present. Once she goes home, if her appetite returns, she may need to be continued on an oral antibiotic. Sometimes, an appetite stimulant can be helpful in getting kitties to eat.
I hope this has helped, please let me know if I can be of further assistance.