2017 Nissan Titan XD diesel S model. Was in town and about 580 miles on the truck the mpg dropped a ton from about 17mpg to around 6-7mpg. It stayed like that for about 10 miles or so then went back to what I was averaging before. I was getting a smell of diesel inside the truck, but had the window slightly open each time I noticed. Checked the level of the def and no change in the level. No lights or warning on dash came on saying that a regen was in progress. Could this be a S model thing to where no warnings pop up saying a regen is going on? And if a regen was going on shouldn't the def drop in level?