Been in a similar situation. My Golden Retriever Maggie developed severe skin leisions that made her smell awful and caused incontinence and other problems. I think it was phisteria poisoning.
I didn't want to put her down but nobody wanted to be around her and her life had become a living hell as it was affecting her liver. No one could find a cure or reven tell us what it was. I held her as she breathed her last and collapsed in my arms. The vet said she didn't feel any pain, I know I did.

*Hugs*