Did you happen to have a recent cold spell abruptly interrupted by a warmish or hot day? That occurred here, and a young hen just about died from heat exhaustion from the wild temperature swing from cold to hot. She came in to the run from free ranging and collapsed at my feet into semi-consciousness. I whipped her up and ran into the house with her and tubed room temp Gatoraid into her crop, a good half a cup. She revived in less than an hour and by evening was back to normal.
Gatorade or Pedialyte are great! Sav a Chick has a pouch of powder electrolytes or probiotics you can mix up as you need, always keep some on hand. Try these, too, it can't hurt.
You know what, another weird one is fungal, not all fungal issues act the same. Sometimes, you don't see the typical fungal signs, they can get slow crop, that's not necessarily sour, but doesn't feel quite right, like soft, but kinda mushy, hard to describe, but my hen had stuff in her crop, not much, but when I felt it, I could depress the mushiness feeling & it didn't bounce back right away like it usually did, which indicates dehydration...like the whole bird was sluggish, pale, weak & her crop was just odd. Her body temperature seemed oddly cold, too. Those couple times, the metronidazole and baking soda did not help. So I figured, here we go, guessing....so then I tried something for fungal, miconazole nitrate, a fungal ointment I sometimes applied to red Roosters bottom (still can't figure out how a Roo can get what resembles a red skin diaper rash skin & no feathers there, when he doesn't wear diapers). Just about as much as a pea, on my finger tip, then gently into the mouth & they do swallow it easily. What amazed me, just a small amount helped within 2 hours. I gave them electrolytes in water & the fungal ointment "pea" every 4 hrs & they bounced back quickly. Yes, they were losing feathers, too, more than normal molt, it was very odd.
When we can't get to a vet that has avian experience, sometimes we just have to try something that may help. It's not good to just guess, but ask any bird "mom" & they'll agree, you rack your brain thinking back to all you've experienced & then reach out to people on this site with mega-experience...no one wants to just give up because there are no decent avian vets nearby.
It's interesting you mentioned she perked up after a bath...every single bird I've ever rescued perked up after a bath, especially ones with those bloodsucking mites, wow they were so relieved & grateful. They really love a hairdry on low, too...practically fall asleep on the towel in my lap when I dry their fluffy feathers after a lavender oatmeal bath...they know what feels good!
I set up a mini hospital in a huge garden tub that never gets used anyway, in my bathroom. We take showers here, so that tub has become the official "chicken spa"
I put some large clean chicken towels lining the bottom, set up food & water, and a box on it's side also lined with cushy towel, and the patient can be monitored, kept warm & have peace while recovering. They do need warmth when they don't feel well.