If this has been going on for a while (sounds like it has) then based on my experience, I might be inclined to go directly to a specialist (internal medicine veterinarian). Especially if the symptoms have not been alleviated by the regular vet within a reasonable amount of time.
Over the years I have found that sometimes it is cost effective, as the regular vets are not cheap either and often they don’t have the expertise or the equipment to diagnose.
I recently had a traumatic event (injury) with one of my dogs, went to the regular vet immediately, times two, only to be referred to the specialist. So, I feel like that money spent was in vain. Plus, the dog was uncomfortable for 3 additional days.
Next time, I will go directly to the emergency vet.
PS: I understand your financial concerns. There is never a good time for these things to happen.
