It certainly cannot hurt to ASK the vet about Mirtazapine. It might be that is is contraindicated with what she is on already, but it is an excellent suggestion. I have seen dogs that would not eat do VERY well on this medication. One was mine who was in renal failure...it settled his tummy and increased his appetite. Another was Lola as Manalos Mom said, and the other was a pup with severe hemolytic anemia. She was not eating and being force fed...and was getting transfusions. She needed to eat and they could not find anything to feed her....they started her on Mirtazapine and she was eating everything! Her mom joked that she was eating fuzz off carpets.
Here is a link about the use of Mirtazapine in dogs...it has been used for years in cats:
Mirtazapine