Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am sorry to hear that Ruffles is getting very thin even with a good appetite, is drinking excessively and sometimes urinates in inappropriate spots. Old age doesn't necessarily cause these symptoms, but it does increase the odds of organ systems breaking down or disease processes developing. While I wouldn't encourage lots of invasive measures at her age there are likely some things that we can do to improve her quality of life depending upon what is going on.
When we see weight loss with an apparently normal appetite we need to worry about disease processes that either don't allow her to digest and/or absorb the food she is taking (such as diabetes, liver disease, primary intestinal or pancreatic disease including cancers) or those that lead to a higher metabolic rate such as hyperthyroidism or cancer.
In an older cat the more common diseases we see include hyperthyroidism (a tumor of the thyroid gland that overproduces thyroid hormones) or since she is drinking more and urinating more, diabetes or a combination of hyperthyroidism and early kidney failure.
Both diabetes and hyperthyroidism are treatable or at least manageable. And so is kidney failure.
Hyperthyroidism is controlled with daily oral medication (Tapazole, also called methimazole) or a very special diet called y/d both of which must be done for the rest of her life or treatment with radioactive iodine to selectively kill tumor cells which is usually a one time therapy. There are pros and cons to each therapy which your veterinarian can discuss with you after getting a firm diagnosis.
Diabetes is controlled with diet and insulin therapy.
Likewise kidney failure is controlled with diet, and sometimes supplements and fluid therapy.
I recommend a a complete blood count and biochemistry profile with T-4 checked as well as a urinalysis to start. That will give us a pretty comprehensive look at her health and allow us to make good decisions about how to proceed.
If those tests are not diagnostic then it is time to move on to more specialized testing if you want to proceed.
If so I recommend checking vitamin B levels to look for signs of intestinal disease, a test for pancreatic insufficiency called a TLI, and an abdominal ultrasound to look for signs of changes in the walls of the intestines as well as evaluating her organs. She may need an endoscopy to collect biopsies of her intestines if they look abnormal.
In the meantime you might try feeding her a higher calorie prescription food called Hills a/d or Iams Maximum Calorie from your veterinarian and make sure she has access to plenty of fresh, clean water.
Please let me know if you have any further questions.