I wouldn't be surprised if many RI chowders now have tomato. For much of the last century the most popular chowder in the state was probably that from the Rocky Point amusement park and included a canned tomato product (as well as savory herbs, sand, bits of seaweed and whatever leftovers from diners' plates the teenaged staff threw in). My old Yankee and RI families refused to consider that to be chowder.
If you think about food history, though, tomatoes have a very brief season in New England while fresh clams and fish were available virtually year-round on the coast. Chowder, like a lot of old New England cooking, would typically rely on cold storage vegetables like onions and potatos, along with salt pork or butter. Some old recipes used common crackers or pilot crackers (not saltines) as a thickener. In season cooks might add fresh vegetables, but they would not be core to a chowder.
Add.: Nice article WJ. Our posts crossed so I hadn't read it before posting this. BTW, my family also didn't consider Rocky Point chowder to be food. It was in the same category as NY System wieners, something forbidden you learned about from the bad kids.