Loud boom and billowing smoke need help diagnosing

Ok, not a newbie but gonna sound like mine. Series 60 Detroit 99 I think, just had elbow and flex pipe replaced on rear exhaust. Jd oil in the morning but was a little off. Drove 50 miles to load, got loaded and 2 miles down the road I heard a loud boom by the fifth wheel somewhere like I blew a bag so I stopped and so much grey smoke was billowing out of the back of the cab underneath that I couldn’t see anything and it was I the truck . I thought it was on fire so I went and turned off the engine and it finally stopped. Now there No oil, no remnants of oil, a few spots on the rusted manifold but nada. Boss thinks I made it up and let it seize up by not checking it. I check it every morning. Any ideas? Mechanic can’t get to it for several days to diagnose. Thanks
Btw…The boom was so loud and there was so much smoke that someone the hwy emergency breakdown guy

 

Smoke was grey and was from the base of exhaust. He said turbo was fine and power was there before loud boom.

 

He says I’m making up all of the smoke and that I haven’t been checking the oil(I do daily) and that I let a ten gallon oil reservoir go dry and that’s what did it. Would’ve had to leaked out in 50 miles and engine was ok power wise. It’s a real old truck but seemed to run ok to the pickup. Thinking his exhaust guy found something and he knew it was a rebuild so he let it blow up on me so he can say it was something I did.

 

So turbo would explain oil loss but gray smoke? Why on bottom of exhaust? Thank you all so much,