17 year old dog can no longer walk?

My dog is 17 this year. She can't see or hear. She eats well and drinks but she can no longer use her back legs. She tries hard to get herself up. I pick up and hold her back legs so she can go to the bathroom and I hold her head up to eat and drink. The vet took lots of tests and xrays and she doesn't see a big problem in the xray with joints. The blood tests results don't look too off so she thinks maybe its neurological or something else going on. I don't like to see my dog trying to get up but I would hate to euthenize (sp?) her because she seems to want to get up and still eats. What do I do!?