3-5 MPG from a plug in module? I've been around diesels since the first 01 Duramax and never seen one of these work that well.. Did you hand calculate the mileage?
Welcome. As far as your question.. some are reporting a decrease in MPG.. and honestly, if you're not happy with the truck I would look into trading it in. You can tune it to get some of that back, but then your warranty is likely gone if/when you need it.