x2
Sorry for your loss.
Hopefully the necropsy will indicate something informative. Be aware, they don't always, but it is good that you are having one done in case it indicates what you may be dealing with flock wide vs. singular bird.
LofMc
Eggcessive has good suggestions...I've considered Coccidiosis as well...you might try Sulmet. That treats coccidosis and as a sulphur drug would also treat gram negative bacterial diseases such as Coryza.
I've used Sulmet when I didn't know if it was coccidia or bacterial in a "hail mary" pass...
Back reading a bit more...If this were vitamin deficiency, she would have rallied with the Electrolytes/vitamins...you would need to double dose the first day or so...but since she is getting worse...I suspect something else.
Try the Duramycin. If it is bacterial you will see some improvement...
Sounds like either viral respiratory or bacterial MG.
I would put her on antibiotic asap. Tetracycline is good for MG/MS. You can generally find Duramycin 10 in the feed store.
Gasping is not usually a Marek's thing, not until end stage if the tumors are lung related.
This came on quickly...