If you are going to try tube feeding make sure to get the tube in the right place. Watch some YouTube videos there are some very good ones that show how to do it.
I would keep her comfortable and warm. I think she has been internally laying or may have water belly. I am sorry. I have lost some of my chickens from both.
You can add Apple cider vinegar with mother to her water. I use 1/2 tsp to 4 cups of water. Chickens like the taste so she might drink more water. I would also keep giving her calcium.
If one hen turns up to have worms you would deworm them all. If you can't find a vet to do a fecal test you can get a fecal test from Amazon or Petsmart. The tests are not expensive.
Google Penelope the chicken from New York. She survived a mareks flare up. They used St. Johns wart and colloidal silver. They also made a sling for her to stand in. Give her a chance but if so is telling you she doesn't have the strength do the humane thing. We just lost a hen from this...