I am sorry to hear you are worried about Shorty. I am a veterinarian and will do my best to help.
Sleeping in unusual places, and shaking, both suggest to me that he is hurting. I know a bed sounds more comfortable than a bathtub to you, but dogs with arthritis
become afraid of being jostled when sleeping, and they often seek smooth cool surfaces to lie on.
Pain in dogs is subtle, they don't use their voice to complain like we do. A change in sleeping places, and shaking, may be all they show.
I would consider fish oil, glucosamine, the herbal product DGP for Doggone Pain, and possibly prescription anti inflammatory and pain medication. At his age, arthritis is likely, and he may seem like a new dog if you start treating him.
A good physical exam by a veterinarian is always a good idea when a dog has a sudden behavior
Let me know what else I can answer or help with.