Have you received a definitive diagnosis? Like someone mentioned diabetes would be a concern, but your vet would probably pick up on that quickly. I would also worry about Cushing's disease, but the part that doesn't fit is the lack of appetite. Cushing's usually causes an increase in appetite and noticeable weight gain in a short period. You can also see low thyroid levels, causes loss of muscle tone so the potbelly appearance. It is usually diagnosed around 9 or 10 years old. The low gravity urine test would fit, but that is just because of drinking large amounts of water. Here's a link for you to review.
Cushings Disease In Your Dog (Hyperadrenocorticism) What Happened And What You Need To Do