I wish I could comment on the medication itself, but I have not tried Tylan powder. In fact I was unfamiliar with it and looked it up on the Foster Smith website (looks good?). I can understand not wanting a dog to be on antibiotics long term, but if it's working and she's a senior dog I wonder if the benefits might outweigh that? It's nice they gave you something that works. My paralyzed dog, Dolly, got diarrhea from "bacterial overgrowth" and they put her on 10 days of amoxicillin and it worked wonders. That was not long term though, so I see what you're saying. I just looked at my old post about it and got a laugh from the title but it wasn't so funny at the time. viewtopic.php?f=5&t=18753
I noticed she is on Previcox, and that can cause GI symptoms. You might want to look up Previcox in dogs and see if you think that has anything to do with it?
You mentioned previously that you think the digestive issue might partly be caused by things not getting moved through her system as well due to inactivity. I would think some added fiber might help with that. My down dogs have all done really well with Science Diet w/d dry kibble. The canned variety of w/d did not work as well. With the w/d dry they have good stools that pass through as they should, and they are not soft and do not make a mess in your pet's bed, the carpet, or your pet's coat. It's been a godsend. Personally I do not recommend just adding fiber sprinkled into the regular diet. I know of a dog who bloated with just a small amount of added fiber, though I know it is sometimes recommended. I guess you could ask your vet. But if you want to try to make the stools able to transit her system in a more regular way by adding a little more fiber and low fat in the diet, the w/d would be a good thing to try. If it does not work for your dog, Science Diet used to take back the bag and refund your money. I don't know if they still have that guarantee, you could ask the front desk staff at your vet and find out for sure. Also, some vet clinics will give you a ziploc bag with some to try instead of buying a whole bag. It's prescription so you have to get it from the vet.