Also what bcr said, you should have finished all antibiotics. The vet gives you a certain amount and that amount should be all used just like when you go to the doctor. Also I hear you, when I had to take my beardie to the vet for RI, it cost me about $270 or a bit more but I had a full blood panel done as well as the culture since she has never seen a vet and I wanted her to get a clean bill of health. Usually if you finish the meds and you animal is still sick, you can call the vet and just get a refill. That's what I did and if the second round didn't clear it up, then I would have had to go back in for a new culture and possibly switch antibiotics. My guess for you is you started the meds and then after a time your snake looked better so you quit giving meds? If that is what happened then it probably never completely was cleared up. Like I said RI can be a real pain.

I probably wouldn't switch to a tub since your ambient temp in your house is 72F. Unless you plan on using a space heater or something to heat the snake room to around 85F ambient. The set up you got temp wise in the tank is good actually. I personally think that not finishing the meds caused the bacteria to not be completely killed. Also you could call the vet and let the vet know how many vials you got left etc and ask for their advice. They might tell you to go back to the same injections for X amount of time longer and give you a refill for X amount to carry you through. Regardless though, keep the temps up so the snakes immune system stays kicked in and it will also help fight the bacteria. Also the heat will speed up the snakes metabolism making the antibiotics work faster too. Its kind of like creating a fever like in humans when you are sick to fight the cold.