When was the last time she laid an egg? I'm very sorry that she is not doing well and may not make it through the night
I wasn't aware that she was still off her feet, so Marek's is a good guess, or there are a lot of conditions in poultry that mimic Marek's symptoms, but aren't necessarily Marek's. It could even be something genetic, or a vitamin deficiency....some types of cancer....a stroke....anything. For your reference, though...a vet doesn't have to be a bird vet to do a fecal check for you. Ours is not a bird vet, but I made him see the light in doing poultry fecal checks several years ago, and parasites are parasites. He can see Cocci and can certainly tell if there are round worms, tape worms, or a variety of worms. Our vet calls capillary worms "hook worms" but it doesn't matter, I know what he means. (Chickens don't get hook worms). Our vet will also examine our birds for us if there's a need, (which fortunately there isn't usually) if it's something simple...treatment/ meds for worms or parasites, or an injury, etc...are things he will sometimes address for us. If he doesn't feel he's qualified to deal with something, he says so, and I respect that. He has a little knowledge of avian vet medicine, as he's taken a few classes. Most bird vets, in my experience, are not great with poultry. Also you don't HAVE to say it's a bird fecal; you can just say you want a fecal check done. That is, of course, if you get a vet closer to you. I'm sorry you feel like you're losing her. Sometimes we can only do what we can do. A necropsy, if you can afford it, is a good idea. However, in some areas they can be expensive, so check pricing if you do that. Lastly, no, don't use dog/ cat dewormer...I've never found Valbazen or any dewormer besides Safeguard at TSC here, but we use Levamisol which we purchase on line, and there are other dewormers you can get on line, too. Safeguard is excellent and a lot of people use Safeguard goat dewormer for their chickens. It's safe to use and reasonable price.
Can you gently feel inside her vent to see if she has an egg stuck about 1-2" up? Is she pooping? Being like this for 6 days, the outcome does not look hopeful.
More things to buy, but when I have a sick chicken, I like to use Rooster Booster B & K vitamins in the water. It tastes good and it will give them a little boost. Other goodies to try when they're sick: Scrambled eggs, chopped hard boiled eggs, bread crumbs soaked in buttermilk (a favorite, and they get some fluids that way, too), minced grapes (another favorite), yogurt, red jello, bananas, baby food, minced tomato, cantaloupe guts, watermelon, canned catfood. Basically, whatever they like that you can get them to eat...also, you can try moistening her crumbles, if she's eating crumbles.