If this is a viral infection, the antibiotics are not going to be a huge help until the infection is gone. The antibiotic will only treat the secondary bacterial infections that occur because of the lowered immune response due to the viral infection.
If you want to see a vet, BY ALL MEANS do every thing in your power to get this done. Not many of us can or would afford to ge this to happen. You must find a vet that is familiar with the avian world or you are going waste you time and money.
Culling for internal inspection may be the only way you are going to find out want disease you are fighting.
I would sure hate to loose any birds to this. If your one that is dying does expire be sure to keep the carcus and free of blow flies and the other chickens.
I sure do wish you luck. I have lost a number of girls to a disease this year that I am still trying to figure out what it actually is.
You can call your local animal shelter and find out if your state has a program for avian disease control and indentification. This organization will preform a Necropsy (animal autopsy) and may determine the disease you have.
These are state run programs and you may are may not have one near you.