Bayatril and pipracil are 2 antibiotics that i have used for RI's and have been very effective. Unfortunately since you have started to use AB's a culture may not show the particular bacteria causing the infection but it is still worth a try. RI's can be fatal and I would recommend trying another Vet which is more familiar with reptile infections. Considering they didn't even try a culture makes me think they are not very familiar with reptile diseases or AB's. Without a culture you are literally guessing which AB wil kill the bacteria causing the RI. My vet ALWAYS does a culture to treat RI's. Snake lungs are very primitive and drowning from RI's is not uncommon if not treated. I would up the temps to 95 on hot and 84 on cool. Best of Luck!