When I first got diagnosed with lyme, I fought for a while to get them to cover any IV treatment, to no avail. Looking back I think part of why I got denied might have been the format of the appeals letters - it kills me to say this but I think it helps to keep yourself as a person out of the picture, and be as factual as you can, talk about
yourself as if all you are is a patient. The appeals letters written for me (I was a kid) were very much about
how it's unfair not to cover my treatment, how I was suffering, how it's wrong of them not to cover it, etc. I'm not really sure how the process of reviewing these letters work, whether it's an actual person making the decision or what, but the less you make it about
you vs. them, the better. Just hit them with undeniable facts - for me they kept citing IDSA guidelines and their version of facts; I think it might've helped had I just hit them right back with ILADS facts and citing studies that prove medical necessity. It's much easier to argue against a patient's word than scientific literature.
Have you discussed with your doctor how to procede? He/she probably has some experience with this situation and might be able to help.