We have a 2010 42 SKQ
we had that problem, I think it was part of the egr system. I took it to Cummins in Jackson, Tn, they first cleared the codes in the hope that was the problem, it wasn't. He removed the top part of the egr manifold, it was okay, then went to the bottom, all the ports were blocked with carbon, cleaned them out, new gaskets. It now has so much power now.
I'm sorry about not know the technical names of these parts. I just watched the service manager
We also owned an older Monaco, it's been years ago, it had no compression in one of the cylinders. We ended up replacing the motor.