Are you sure that she is eating rather than pretending to eat. I've had several chickens that appeared to be eating but when you looked closely, were picking up and dropping the same bit of food or missing it due to poor vision. Chickens will pretend to be healthy as long as they can because once they show weakness the flock may try to run them off to prevent the spread of disease.
I personally find that production hens would be a 2 at most on that condition chart, but she is a big breed and should be larger. The first thing to do is to isolate her into a crate which has a heat source and where you can actually monitor her food intake and output and check her crop function. Keeping her warm is really important because she has used up all her body reserves if she is so thin.
There are a few things that cause them to waste away. Crop problems are one of the most common, so I would check that out first. Remove food from her cage at night and then check her crop in the morning. If it is still soft and squishy, that may be your problem area. The crop can get clogged with fibrous material like long strands of grass/hay/straw that get ravelled up and form a soggy plug which slows down the passage of food into the digestive tract.
A partial blockage in the gut sometimes caused by reproductive issues can also slow down the crop because waste gets stopped back.
Tumours are another cause of muscle wastage and are all too common in chickens.
Internal parasites are another possible cause. An ordinary veterinary practice can do a faecal float test on chicken poop without having to know much about chickens or I believe there are mail order options to have testing done in the US. Here in the UK I can get next day results from a mail order lab testing business... but obviously you have to buy the testing kit first, which may take another couple of days to obtain.
Those are the prime causes off the top of my head. In my experience mites need to be present in truly massive numbers to take a bird down to this extent and other birds would be suffering too and the clusters of mites would be pretty obvious... and of course, it's not the time of year for mites to breed unless you live in a warm climate, so very unlikely that mites are the issue.
Anyway, get back to us with result of crop examination in the morning before she has had a chance to eat and also check for any abdominal swelling. Keep her warm and offer her some scrambled egg once the crop has been assessed. If the crop is still squishy in the morning, start massaging it (10mins 4x a day) and only feed small amounts of wet homogenous food (pellets or crumbles soaked in plenty of warm water) with no lumpy bits or grains or access to fibrous material.
Please update us with more info regarding state of crop and any other unusual lumps or swellings.... compare with other healthy chickens if you are unsure.