I would discourage the use of multiple antibiotics at once unless it's really called for. Drugs take their toll on an animal too. Otherwise, instead of diagnostics, vets would treat every animal with a barage of medications knowing at least one will cover the problem.
It is important to take a direct, consice route of action against the problem. If I'm not making an improper assumption, you are treating your snake with whatever you have at hand, and not treating your snake with what the problem leads you to. Inappropriate antibiotic use might still get rid of the RI for now, but if it doesn't, or if another infection returns in the future, you'll have only made it more difficult (and expensive) to treat.
My recommendations are:
One, visit the python forums here, grab a book or two and be certain your husbandry is perfect. Doing this will also help you find the answer to your humidity question. Optimal husbandry will not only be your best help towards a cure, but in preventing future problems.
Two, since this guy is already sick, and sounds rough, take him to the vet. If you want, let him know what drugs you have already, and if it fits, he might incorporate them into the treatment plan to save you a few dollars. The benifit of the vet visit will be documentation for reference to future care, a professional hands-on exam, supplementory treatment if needed (ie, fluid therapy or other), diagnostics as seen appropriate, and a treatment plan that takes into consideration the needs of the animal, the risks of plan A versus plan B, and of course your wallet.
Ian