cat poo.. at wits end

My wife and I are desperate for help. We have a 13 year old cat (which we have been pet-sitting for 7 years when she was left with us) and don't know what to do. In the past 2-3 months our cat has lost 5 pounds (from 15 down to 10) and she has been having diarrhea all over the house and has ruined the carpet in 2 rooms (we have been successful in cleaning the carpet in the others so far.) She will pee in the litter box, but not poo, and we keep the box very clean. We have introduced a second litter box near the first, and a third downstairs, but it has not helped. She not only goes on the floor, but anything we may have on the floor (papers, clothes, shoes) We have spent $2000+ at the vet (including a second opinion from another vet) for every test imaginable, including abdominal ultra sounds and x-rays, and they can not find anything wrong with her. We have a small townhouse so we do not have a basement or garage for her to live in. We love our cat, but she is destroying our house and we can not live in poop forever.

Other information that may be useful:
Food: Science diet adult light (recommended by the vet)
Litter: Tidy Cat clumping (we have also tried arm and hammer unscented non clumping)
Pets: We have a second cat (5 yrs old - no problems)
Both cats indoor only

Do you have a copy of the blood work? I'd be curious to know what the thyroid levels were.

I will see if I can dig them up, and then I will post them. Both vets thought it was a thyroid problem, and both said the blood test came back negative for thyroid problems.

What about a viral problem? My friend's cat was exactly like that and the vet said it was a corona virus.

Did your vet give you anything to help deal with the diarrhea? Even if they don't know what is causing the problem they could at least give her anti-diarrheal food..

Is your kitty also throwing up or just having diarrhea? Is there ever blood in it?

I would isolate her to the kitchen/bathroom/other easy to clean area for now..