These guys have really slow metabolism and can survive with much lower oxygen levels then mammals.
That's why they are capable of being shipped in deli cups. That's why they can stay under water for long periods. That's why it is not recommended to euthanized with C02.
My bet is your snake is in severe respiratory distress as evidenced by the post you made last night about the heavy breathing. Listlessness happens when RIs get dire. If there was still tea tree oil residue in the tank that could have been really bad. I use tea tree oil for various things with my other pets and tea tree oil toxicity can be devestating. Severe symptoms include, feebleness, paralysis, decreased consciousness, coma, tremors, seizures, and death.
I would insist that the vet take a blood sample. I'm not sure how tea tree oil would metabolite in a snake but I'm mammals it will elevate liver enzymes. Might be a good place to start to figure out if it is a case of poisoning. I would also insist on a culture if it is an RI to pinpoint the exact organism causing it so you can do a specific antibiotic instead of full spectrum (which tend to be weaker)
If the little guy makes it there may need to be a drastic change in your husbandry and handling practices.
But yeah. They can be pretty resilient. I worked in the exotics department of a big box pet store and have seen a few zombie / jesus snakes.