This may sound like a strange question, but does her stomach look like it kind of sticks out on the sides? A dog I had was much younger than yours and was diagnosed with Cushings Disease. The first symptoms were excessive peeing and drinking. It turns out she was diabetic, and the vet left the testing at that. Well, over the next couple of weeks I noticed her back legs were really shaking a lot and her stomach almost looked like a pregnant cow's stomach. I was online doing research about diabetes in dogs and came across Cushings. Diabetes is commonn in cushings, but the big thing was that symptoms of cushings was excessive drinking and peeing, extended stomach, and loss of muscle tone especially in the hind legs. What made it worse was that several years before that she had surgery to repair a torn ligament in one of her hind legs. I had given her a glucosamine / msm product until she was diagnosed with the diabetes (glucosamine raises blood sugar). Salty foods are really bad for diabetics. One time (before I realized what was going on), she had some ham and peed while laying down. Another time she did this with roast beef. I'm not sure why, because I didn't think it was a salty product, and there was no salt added to it while it was cooking.
Anyhow, it sounds like she has some symptoms similar to cushings disease. To counteract the loss of muscle tone in my dog, I started taking her on many short walks each day. Every week or two I would take her a little farther on each walk. It really seemed to help her legs a lot.
Other symptoms are that the dog will get hot. Does your dog seem to lay on cooler surfaces like tile flooring or stuff like that? I got a canine cooler bed for my dog. It stays cool, and she eventually figured this out (took her a while to lay on it) and she ended up loving it. Also, another symptom is restlessness during the night. Does your dog seem sleepy all day and then maybe pace or wander around at night?