Exhaust smell

Might be a cracked pipe or piece of flex near the turbo somewhere would be my guess but wouldn't rule out anywhere without a close inspection. Look for tell tale trails of soot.

 

Do you drive with your window open?
Do you have weed runner exhaust , or regular stack?
My 2016 Cascadia has two black plastic vents on the back of the cab. The idea is for you to drive with the windows closed. The HVAC system pressures the cab and the air flows through the heater/ac vents, the cab and out those plastic vents outback.
I run a lot of gravel roads, I found out that if I drove with my window open, then road dust would get sucked back in from those rear vents and the cab would fill with dust.

 

On my ProStar, I was having an exhaust leak like you are experiencing, I fixed it after discovering that there was supposed to be a gasket in-between the plumbing at the turbo. The dealer said no there isn't, and then I gave them the part number. I fixed it myself.

 

Advice above to look for soot is sound. I have a M-2 w/3126 Cat that had the turbo flange clamp
put back on a little ####ed and it didn't seal properly. Year or 2 later had a band clamp rot thru and was letting exhaust in the same blk. vents on back of cab as was mentioned.
Both times found it by the soot.

 

Check the google ratings of the dealership. Some of them are no better than a TA. Google truck repair shops and try to find one in the area with at least a 3.8 rating.

 

One thing I did do was put a worm gear hose clamp around the top of the shifter boot to keep the engine compartment air out of the cab. I also replaced the boot, which was shot and properly secured it. I have a weed burner too. Replaced the gaskets in the exhaust system and bought all new band clamps. Problem fixed.