First, I am so sorry you went so long without a reply.
It seems you have tried every possible avenue to try to keep your dog hydrated. And I guess nothing works? All I could possibly suggest is what you have already tried!
My dog also went through phases when she was uninterested in drinking water. I usually made her a soup/stew with meat or fish and vegetables and give it to her twice a day whenever that happened, so at least I knew she was getting basic sufficient fluid intake.
With my dog, she would suddenly come out of it, and start drinking her water in an ordinary way again for a while. I never had any idea why that happened. All I could do was manage it as best as I could.
But your dog not being able to gain weight....how bad is that? Is she staying at a pretty decent weight, not looking too skinny, just "slim"? Or is she beginning to look wasted and you can literally see her bones? Is she also acting fit and well, full of energy, or is she lethargic, without as much energy?
It could just be that she is burning off all the calories she eats by her daily activity.
Or, it could mean she needs a full blood count, just to check nothing more serious is behind that inability to gain any weight.
If her blood work and vet exam is all fine, then maybe she is either genetically inclined to be slim (I know this because I am the same, and take after my father. I cannot gain any weight, though I don't lose it.)
Or she is burning off the calories in the food she gets. And if she is a very active athletic dog, she may need a slightly richer food.