I haven't had that issue with an adult dog and it does sound like it would be upsetting to all involved. My only thoughts from your post run to the following, which are more to treating the symptoms than learning the cause:
You said she's drinking a ton of water. Is this more than usual for her or does she need it to keep from getting dehydrated from the diarrhea?
The Royal Canin Gastro is very low in fiber. My daughter's dog, who had irritable bowel syndrome, was put on a fiber supplement to try to absorb some of the extra fluid in her gut and slow the passage of food through the intestines. I've also given it to a puppy with diarrhea. It helped my daughter's dog but didn't seem to have much effect on the pup. The dose can be a little tricky to manage- too much fiber can have the opposite effect. My daughter's dog was getting 1 teaspoon sprinkled on her food at each meal, so twice a day. You could ask the vet if adding some fiber to the diet might help. Many people recommend pumpkin as a fiber source but you'd have to give a large amount of pumpkin to get the same amount of fiber you could get with something like Metamucil (psyllium) or Benefiber (wheat dextrin). My vet said it didn't matter which fiber you use. This is the article about using pumpkin: 403 Forbidden
You didn't mention what antibiotics Tressel got. For a while my daughter's dog was getting a small amount of Tylan powder sprinkled on her food daily to try to ward off diarrhea- that was early on before she was formally diagnosed and other meds came into play. My limited knowledge of Tylan is that it's a pretty safe antimicrobial that can be given long term for diarrhea. Metronidazole is frequently given for diarrhea as well, maybe she's already tried that.
I know you'd rather find the cause, not just treat the symptoms, but managing the symptoms in the meantime sounds like it would be welcomed. Let us know how things go, and I hope this gets straightened out soon.