Heading west from MA on route 90 and rte 80 ending in SD wondering where EZ Pass stops working. Called Ez rep and was told. "It works up to the Mississippi river" what happens after the Mississippi ...
The best answer I can give is to scroll down this page on the E-ZPass group website to the "Where can I use E-ZPass?" heading and look at the little portion of a map there.
At least in the cases of Vermont and Connecticut, the lack of E-ZPass integration is due to a lack of toll roads or bridges in the state.