Dogs compensate for weakness long before they stop walking, so the early signs are easy to miss.
Watch for a swaying back end, knuckling of the rear paws, difficulty rising from the floor, a hunched stance, or a dog that used to lead the walk and now falls behind. Scuffed nails on the hind feet are often the first physical clue.
A wheelchair is not a last resort. Used early, it keeps muscle on the front end, protects the spine from compensating, and keeps your dog exercising while the rest of their care plan does its work. Talk to your vet, then measure rear leg height and start there.