We've tried Pet Naturals of Vermont - Calming XL. They are natural but I don't know if they'd work for that level of anxiety. We used to have a very old Siamese cat that would howl at night, he seemed very disoriented and was going deaf.
Sounds like sundowners syndrome in humans. Does it only happen at night? If so could you leave a night light on or see if melatonin is safe for dogs?