I’ve seen old Yankee and RI chowder recipes that do not use tomatoes and I’ve had RI chowders without tomatoes. IMO when they are present they should be firm pieces and not cooked down so that they color the broth a deep red.
Growing up I was told that tomatoes in RI chowder were a "new" thing (i.e. early-mid 20th Century) probably from the strong Italian influence in RI.
Manhattan style can be a good seafood soup, but it doesn’t cut it as chowder in my book and once you’ve had a good Portuguese seafood stew or a good bouillabaise Manhattan chowder tastes like tomato soup.
As for thickener, I agree with seafarer john. I think most commercial chowders use too much flour or roux (or other things like corn starch) to thicken and whiten. Chowder should be thickened with potatos, only.