Whoa, hold on a second.
She's showing no signs of upper respiratory illness (mucous, eye irritation), and even worse, that gurgling sounds lower. Unless the vet showed you how to do it, don't put any liquid down her throat, you can very easily make her aspirate it, then you will have a real problem (no organism can breathe in oil - that's how every case in the "vaping crisis" happened, CBD vaporizer fluid was being cut with vitamin E oil by multiple companies, and apparently no one knew or cared that it would literally kill their customers).
It's not hard to tell at all! A throat swab sent to (any) state lab and they can have PCR run and results back in a day or two! Hell, I can set up PCR in my garage with stuff from the hardware store. That Vet straight out guessed based on what's most likely to be active right now and not the actual symptoms or a test.
My rooster made a similar noise after he aspirated himself from drinking too much water because of an impacted crop. I have half a dozen teenagers in sick bay right now because they actually do present like they have MG, but they do not, it's a bad reaction to environmental factors (e.g., fire, ammonia, and one ate a bee).
Do the VetRx and follow the instructions on where to put it/how to administer that came in the box. If the Tylosin doesn't work and no more get sick, it's not Mycoplasma, but if it is, he should have written you a prescription so you could have gotten a whole bottle to keep for future outbreaks. Because it never goes away. Ergo, he was either playing you or catering to you, but not practicing sound medicine. By only giving you 5 days worth, I'd guess he was catering to you. Mycoplasma is quickly becoming antibiotic resistant, it is not cool to throw tylosin at something that's not confirmed MG. If it's some other bacteria, the Tylosin won't work, and you'll have wasted a week while the other bacteria grew stronger, and if it's in the lower respiratory system, it'll probably be too late.
You're not going to be able to lower the humidity in the coop much more than the outside humidity, so in that sense, your local friend is right. Bringing her indoors where you have heated air should lower the humidity enough to help her clear up and help the VetRx work. Give her clean water at all times, you can add the VetRx as recommended in the package insert. If she's eating, make sure she has grit too, just in case it's something like impacted crop. (I doubt it, but people often overlook grit when putting a bird in quarantine, feeding their same food that requires them to have grit.)
How clean is that coop? Is it dry enough that you stir up dust every day when you clean it, are they in there when that happens? Immunity only happens when our bodies have something to fight off, keeping it too clean can have the exact opposite effect from its intention.
Birds have much more sensitive respiratory systems than we do, and less defenses too. Evolution didn't get the memo that chickens can't fly, so they still breathe in twice as much oxygen than other animals. They actually take in oxygen on the inhale and exhale, and don't have a diaphragm like we do, even their bones are critical to their respiration. Spraying any chemical (including perfume or the stuff coming out of your dryer vent if you use fabric softener), stirring up dust, PM in the air from fire or pollution, any of these things is enough to give a bird a chest rattle. The one thing that amazes me though is how many times I've seen a pet bird in a smoker's home that hasn't keeled over.
Aquarium fish and birds are the two animals that I most often see killed with kindness. They are only helpless and vulnerable because they are trapped in a cage, at your mercy. They do not have these problems in the wild. Keep an eye on her, but from a distance, unless an infection sets in, she will probably resolve on her own in the dry inside air. You'll know within a day or two whether it's something that's going to clear up on its own or not. If not, go back to the Vet and have the labs run. Tell them to call the lab if they need assurance that all the tests can be run before the animal dies. Millions of people sticking cotton swabs up their noses everyday can prove it.
