I would concur with your veterinarian. The symptoms you report are non consistent with FIP. Therefore FIP nosodes are not going to be beneficial.
FIP has a wet form and dry form. If you want to hope your cat has FIP, it could only have the wet form. The wet form has fluid and tiny sand to pebble like nodules in the abdomen. A large tumor would not be present with FIP.
A large tumor may in fact, be present with FeLV or feline leukemia though.
Most likely the tumor is the cause for all of the symptoms. Tumors cause pressure changes and changes in protein levels resulting in fluid in the abdomen. They cause weight loss (also known as cachexia) and muscle loss. Lameness/gait changes are a common result. Constipation often occurs when fluid that should be in the vessels are in the wrong place - it fluid in the belly -less fluid in stools is hard stool = constipation.
It's hard to let a loved pet go. There comes a time in each pet's life that it does occur. If a pet is having more bad days than good, it's time. It's an individual decision, but if the symptoms said here are occurring with out lots of improvement elsewhere - it may be time to let her go. Euthanasia is often a gift of easing a pet's suffering. Consider it and talk with your veterinarian. She would be able to let you know if it's too soon.
Thanks Dr. Mathis:
You are right. I know my Vet has made the correct diagnosis, he always does. He has been treating my pets for the last 15 years. I guess I was stressing a bit, mind was reeling with thoughts of trying to make her better.
I have spoken with him (Dr. Kim) and he said as long as she is eating, drinking we can only try to make her comfortable until it's time.