Unintentional Coal Rollage
 

07.5 6.7, 95xxx miles, no deletes.

My brother in law has been starting it up once a week and letting it run for about 15 minutes for the last couple weeks. Then it sat for about 3 weeks until I was able to drive it again. It started and ran fine, got up on the highway and kicked in the exhaust brake, stomped on it to spool it up and then chopped the throttle to let the brake do its thing. She smoked, like old 12v with 100hp injectors smoke. Is this just shit built up from idling without the break being cycled?

You have issues somewhere. Its probably plugged up from idleing around. Cause it can't regent you should have just let it sit. With the emissions intact it should never smoke.