Did the vet give prednisolone to her for the allergies?

If so, I'd taper her off and see what happens. Pred can cause psychosis as a side effect.

Are you giving her more attention during the day? Some treats of shrimp or chicken?

Do some Googling about feeding a diet for kidney problems. The treatments tend to favor wet food instead of kibble now.

It sounds very sad, your description of the howling which sounds like humans screaming.
We've had cats die of cancer-- no howling.
We had a cat with hyperthyroid disease -- no howling, but extreme hunger and begging for food.
We've had a cat die of kidney failure-- no howling
We've had a young cat with a severe abscess on her neck from an animal bite in extreme pain-- no howling

However, our Maya, did wail when she was young and a kitten and afraid.
And I have had cats really wail during car travel (they were caged).

I am tending to agree with Jo, that this might be a severe grief or stress reaction. Giving the kitty more attention, and loving, may make this better.

I can surely sympathize with you and how the howling makes you feel.

You could also try giving some quality catnip. Some cats settle down with it, and we found it helpful for pain issues too. We use it for traveling in the car.