Totally did it, based on some recipes from the internet.
Mix one can of tomato paste with 4 cans of water until smooth (this makes about 4 cups). Set aside.
Melt 2 T of butter over low heat, add 2 T of cornstarch and mix well to make a roux.
Add the tomato stuff to the roux, turn up the heat and stir until it boils and thickens. It might look horrible until the end, like it has a nasty rash, but eventually it will turn out smooth and pretty.
Add 1 c milk, 1/4 c sugar (or to taste, but this seems to get that Campbell's level of sweetness), and 1 t salt. Stir to dissolve the sugar.
The thickness is just right. Now if only I had saltines...