Hi Nate.
I don't like to use medication for herps at first sign of something wrong but, it's me... I'm not a Vet.
If this snake was mine, I'll go at the lower range of your 65-80% humidity and bump temperature. In fact I would try to raise temp to the higher of it's optimum temperature, with a hot basking spot(120F, even a little higher). I'll try this for a couple days and if it doesn't work, then i'll go with the Dr. recommendation.
I used this technic with good succes in colubrids, boas and monitors a couple times, but like i told you, i'm not the expert.
You probably already know, Ri are a bactery in the lung, generally cause by inadequate husbandry (low temp and high humidity), stress, dirty enclosure, ...
Take care, good luck and let us know how it recover.