Definitely get the sick one to a vet quickly. I had one pop up with an RI earlier this year and the visit + meds was under $100. The vet thanked me for bringing the snake in at the first sign of a problem as it's cheaper and faster to treat the illness early than if you wait; he said most people try home remedies for a few weeks and then only bring the snake in when it's really sick. The other thing is that the longer this one is sick the more likely one of your others is to end up getting sick also. Even if you QT the sick one, if you have central air it'll spread the germs through the house.
ETA: My vet did recommend steam treatments like you've been doing, but without the additives. Just use the warm water, the steam will help the snake expel mucus. While the snake is getting treated, completely clean and disinfect everything in its enclosure - the more germs you remove from his surroundings, the fewer he'll have to fight off.
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