You can get 20% off every day without trying too hard on ANY new truck, including the one that was produced yesterday.
So, I would expect a significantly larger discount on a 2015 just because I would....BUT let me throw out another perspective:
If you're a high mileage driver, or someone who keeps a truck a long time (10+ years) it really won't matter. If you trade every 3-5 years, the two-year-old new truck will beat you down in resale value. But if you keep the truck for 10+ years, or you're a high mileage driver/operator (say a hot-shotter or a full-time RV type) the "Low mileage for a 5 year old truck" think can play in your favor.
I try to balance it. I have no qualms whatsoever about buying last-year's model, in fact I seek them out -- but I'm a high mileage driver (when I drive at all) and I generally keep trucks until they start falling apart...over my life I've averaged a new truck every 12 years....and by buying last-year's model when I do, I've come out OK on the mileage vs. years thing, and on a 10+ year old truck, one MY difference in depreciation is less than the mileage hit.
But unless you fall into that special niche that can benefit from the model-year gap, I wouldn't touch that truck for less than a 35% discount from MSRP, factoring in the normal 20% and basic model-year depreciation.
As stated above, warranty and service would be the same whether you buy the 2015 or a 2017....