How would one unsweeten a can of campbells tomato soup?

I would add a can of crushed tomatoes, a bit of seasoning. This will just make more soup but it will "dilute" the sweetness of the canned soup.Dilute with water or milk depending on how creamy you want it.

To add more flavor I would also saute some onion and add that with the tomato.

Try adding a carton of passata,this should dilute the sugar without spoiling the tomato-ey taste and texture ( will also add more tomato, good for your 5 a day)

I don't think you can take sugar out of something like condensed soup. I love it, but if you find it too sweet, making your own tomato soup is pretty easy.

You could add lemon juice or vinegar to counteract the sweetness. One teaspoon at a time.