Dog with diarrhea lasting one month

[Renordw]

Species: Canine
Age: 12
Sex/Neuter status: Neutered Female
Breed: Shiba-Inu/German Shephard Mix
Body weight: 35 lbs

History: She had oral melanoma in 2012 that was totally removed via excision of the affected tissues, and did not metastasize. She has taken several treatments of Oncept (four in total), and the cancer never returned. She has arthritis in the feet and lower back.

Clinical signs: Very frequent runny diarrhea (5-6 times per day) and weight loss, no normal stool
Duration: two months
Your general location: Southern New Mexico

The vet has stated that the probable cause of the diarrhea is bowel cancer, however at least in humans, melanoma does not normally metastasize to the bowel unless it is also present in other organs as well.
Currently, the dog is mostly healthy and happy in most every other aspect, except occasional limping from arthritis, the obvious problem is that she is wasting away.

I need some kind of ideas, she's currently on prednisone and neurontin for neurological pain.

One fecal transplant was performed at home via enema from a healthy dog in our ownership, and within six hours, she regained her appetite and her spirits greatly improved, as well as her appetite. However, her diarrhea did NOT subside. Her weight has stabilized.

[Renordw]

Sorry. I was unable to edit, after accidentally posting. I am looking for any additional ideas. We've had about 800 dollars worth of every kind of test done, including parasites, blood tests for cancer. Nothing has come back positive.