Unfortunately, I would agree, at her age and with the symptoms described and that photo of her posture, it is almost certainly Marek's Disease. If/when they stop eating and drinking I euthanize them because they go downhill rapidly after that. I've given supportive care for months to birds which were still eating and had some recover from an outbreak even after than length of time but not if they won't eat and trying to force them risks them aspirating fluids and it just stresses them. I'm sorry I cannot offer any advice for this bird except to help end her suffering.
She will be actively shedding the virus whilst she is symptomatic like this.... it is shed via dander dust which is inhaled to infect other birds but there is a minimum 3 week dormant period between infection and outbreak although it can be longer, so you may see other birds develop symptoms similar to this in the coming weeks and months. I would recommend you get a necropsy to confirm my suspicions when she passes. State agricultural or veterinary diagnostics labs are the place to contact for this service and she would need to be double bagged and refrigerated (not frozen) and sent off or delivered to them asap.
I'm really sorry I can't give you more hope for this girl. I would probably give her a couple more days to see if she will get her appetite back. Try feeding moist food like scrambled egg or even bread soaked in beaten egg or berry smoothies to tempt her and a good poultry vitamin and electrolyte supplement but if she won't eat there is no hope and the chances are she has a tumour(s) growing inside her caused by the virus.
A chicken sling in the meantime, may make her more comfortable. They can be made from everyday items..... see below
You need to adjust it so that her legs just touch the bottom otherwise she may panic and you will probably need a wider strip of fabric to support her back end with a third hole cut in it under her vent for poop to drop through. If she is not happy in it though do not persevere as stress will make her worse. You can clip pots of food and water to the front of the box so that she can eat and drink herself if she picks up.
I wish you luck with her but sadly don't hold out much hope. If you can put her outside in the sunshine on grass, down wind of her flock mates in a secure cage or pen that can sometimes give them a boost. They often get depressed once they are isolated. Sadly however, a high percentage of birds with Marek's do not make it.
