Ovarian Cancer: The patient's presentation with loss of appetite, significant weight loss, ascites (as evidenced by a 5L paracentesis), and an elevated CA-125 level of 1200 is highly suggestive of ovarian cancer. Ovarian cancer is known to cause these symptoms, especially in postmenopausal women, and the elevated CA-125, while not specific, supports this diagnosis in the context of her clinical presentation.

Tuberculous Peritonitis (TB Ovary): Although less common than ovarian cancer in this demographic, TB peritonitis can cause ascites, weight loss, and an elevated CA-125. The patient's symptoms and the fact that TB can affect the ovaries and peritoneum make this a plausible diagnosis, especially if there's a high prevalence of TB in the area or other risk factors for TB.
Other Malignancies: Other abdominal or pelvic malignancies could also present with similar symptoms, including but not limited to gastrointestinal cancers, although the elevated CA-125 might be less commonly associated with these.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can present with systemic symptoms like weight loss and loss of appetite, and can involve the abdomen, causing ascites. An elevated CA-125 can be seen in some cases of lymphoma.
Severe Infection or Sepsis: Although the patient is on immunosuppressive therapy (prednisone and fludrocortisone), an infectious cause of her symptoms, especially if she has a compromised immune system, must be considered and ruled out promptly due to the high risk of mortality.
Drug-induced Causes: The patient's current medications could potentially contribute to her symptoms, though this seems less likely given the specific combination and the presence of ascites and a significantly elevated CA-125.

Pseudomyxoma Peritonei: A rare clinical syndrome characterized by recurrent mucinous ascites causing abdominal distension, which could be considered in the differential diagnosis of ascites, though it is much less common than ovarian cancer or TB.
Mesothelioma: Primary peritoneal mesothelioma is a rare tumor that could present with ascites and abdominal symptoms, though it is much less common than ovarian cancer and would typically have a different marker profile.