My 12 Yo Crossbred(kelpie, Cattle Dog) Dog Has Been Losing

My 12 yo crossbred(kelpie, cattle dog) dog has been losing control of her legs over the past 3 weeks, has no strength in her hindlegs ( trouble standing, climbing stairs), has trouble walking (unsteady) and has little appetite and does nothing physical. She has been to 3 vets and had blood and urine tests which did not show anything major. She has had an antibiotic injection (10 day). She is on pain relief but it does not seem to help her symptoms. She wants to do things like walk but is too weak. Thank you for your help