Based on your description of how the hindlegs are being impacted, the conditions that would come to mind first (in no particular order) include:
1. Arthritis! Absolutely, a important differential.
2. Diabetes. It's called a diabetic polyneuropathy that can cause irregular hindleg weakness
3. A disc herniation somewhere in the spine.
Great that eating and drinking normally. I think a vet visit to get some x-rays of the hips and lower spine might answer a lot of questions! In addition, at 15 years of age, a basic blood/urine test would be invaluable to evaluate overall health and organ function.
It's quite possible Natteman could do very well on a long-term non-steroidal anti-inflammatory like metacam, or a drug called Onsior that your vet may carry. Also, a little pain management trial with buprenorphine might help as well.
Definitely, maybe some options here, but you'll need a new vet visit to discuss the above.
Hope that information helps