First off raise the temps a few degrees and keep them there. Lower the humidity also. That will Kickstart the immune system. You can also get a repti fogger and f10sc and a tub to nubilize (spelling?) it. The vet should either do a mouth swab culture or a lung wash. If they don't and just prescribe a treatment then they are wrong and you need to tell them so. An RI can be bacterial or fungal and it could also not even be an RI at all.