A good friend on mine suggests something else a little unorthodox but she says it helps. She is a zoo keeper in a reptile zoo so I tend to listen to her. She suggests as odd as it sounds cat nip plants. Live cat nip plant that is. Catnip has been long used to treat coughs and has a mild antibiotic effect too. She claims it adds oxygen (it is a plant) and the fumes from the leaves help to open airways. I think it makes some sense. She does not claim it is a cure just aids the snake in breathing freely.
The next question is are A you sure it is RI and not just some water in the mouth. RI fluid is very slimy, not like spit but like phlegm.
The second question is have you gone over your husbandry very very carefully. Reptiles do not normally just get RI it comes from a depressed immune system and that usually points to a husbandry issue. Have you checked your temps all of them ambient air temps too? Do you have enough ventilation? Humidity is it in a reasonable range 55-65%? If there is an underling issue you will not get rid of the ri it will persist.