You know when you have a feeling about something and you can't shake it. You're convinced something isn't right with your dog, or you notice something that isn't normal. These can all point to medical implication and when there is a medical component to any behavioural concern you cannot proceed expecting positive change to behaviour until it is under control.
If you have medical concerns, any vet worth their salt will take you seriously and do appropriate investigation if needed. It might be a quick blood test for rule outs in organ function or other things that they show (I'm not a vet). It might be requesting they take your concerns about pain seriously, and then the answer may be a pain trial if they don't have any overt symptoms. If your dog has suspected musculoskeletal (bone and muscle) problems (think hips, back, neck etc) then perhaps some scans are in order. Any decent vet will be willing to entertain these worries or concerns.
Behaviour is inherently influenced by our health. Dogs, people, cats, elephants. Any living creature is impacted by poor health. Lethargy is behaviour, hiding away in our beds and watching tv while we recover from a cold, is a behaviour (well a big old clump of them). A dog not wanting to be messed with and growling or biting when they have pneumonia and you're trying to do stuff with them is a behaviour exacerbated by medical interplay. Ignoring your dogs medical, or your vet not taking it seriously can have severe consequences.... not only that your dog feels like crap. Their behaviour can always escalate. For this reason your vet should take you seriously.
What if they don't?
Find a new vet.
Simple.