Do not lower humidity with a snake with an RI; you want to bump up both its heat AND its humidity. Think about when you are congested and take a hot, steamy shower; a lot of the crud in your sinuses and throat breaks up and you can expel it and breathe more easily. The same goes for your snake.
Interestingly enough my vet doesn't do cultures unless the client specifically asks; he pushes both injected baytril and fortaz instead. His reasoning is that the culture is more expensive than the meds, the vast majority of RI's will respond to one or the other, and people with less valuable snakes won't bring them to the vet if a RI costs $300-350 to treat instead of $150 - they will just let the snake die and get another one.