My dd15 Cascadia125 MPG drop too much!Can't find any reason!

I've heard of soot buildup in them things causing problems. I assume it has a variable geometry turbo, they can soot up or function poorly as well. Fairly easy to look at the intake to see if the egr is full of soot.

 

The one box could be packing up with soot also, restricting exhaust flow.

 

when is the last time you greased the clutch release bearing

 

He said it was AT trans?
Pull your delta p sensor. Check venturi ports for soot build up/plugging.

 

They still have a clutch unless it's an Allison.

 

If the truck has sealed s-cam tubes they may be drying out so the brakes are not fully releasing. If it has that style they can be greased.
I would suspect a wheel end problem of some sorts seeing as it is dragging as you say.

 

I'm aware but being a 2017 it is most likely a DT12 which doesn't have a greasable throw out bearing

 

it could be a lot of things but the OP description sounds exactly like my eaton 10 speed auto throw out bearing screaming for grease. loaded heavy it's hard to tell, load momentum masks the symptom, but after delivery, pulling the empty, when you let off the throttle there is a unreasonable amount of highly noticeable deceleration, due to clutch release bearing not disengaging from engine. a few pumps of grease into clutch release bearing, no more deceleration on throttle release, it coasts forever just like if it was in neutral. but could be brakes, hubs. if it was brakes and it didn't get better or different over time, there would be smoke at some point. if it was a hub, for there to be that much friction would mean also a lot of heat, you would be able to see smoke or feel heat if it didn't get better through the day. but clutch release bearing not letting go of the engine will do the toddler-attached-to-pants-leg symptom without any other noticeable symptoms. either way, something needs lube!