First thing STOP bathing your snake, bathing snakes which increases stress + improper husbandry is a bad combination, bath even without improper husbandry is still something that you want to avoid.

Second with a RI a culture MUST be done, hence the need of a good herp vet, a good herp vet will do a culture before prescribing any type of antibiotics, not all RI are the same.

Third the usual treatment when diagnosed properly is injectable anti-biotics and should last a month.

Not all vets have reptile experience and that is really key when taking your snake to a vet.

Right now go back to your vet (only if you believe that he will take what you will tell him in consideration) or find another one, get a culture and get the proper treatment, limit stress (that mean no handling except for maintenance and treatment) also remember that feeding right now is not a priority once a week on schedule if he takes it is more than enough, IF the animal is underweight this can be addressed once the animal recovered.