Yep; just smaller displacement. ISX-15 (KA/Essex/LA) is a 15-liter engine rated to 600-605 HP while the ISX-12 in the MADP is a 12-liter (actually 11.9-13 liters, depending on configuration) rated at 500 HP. They were replaced in 2019 with the X-15 and X-12versions. Pretty much the same engines as the earlier versions, but a bit lighter.
While at Cummins for my recent head gasket repair I inquired about the ISX (assuming the 605 of course). They didn't speak highly of it as they said it's too much for a RV, it runs too hot. Also had the piston issues in the past which they said Cummins corrected. Of course this is one service tech's opinion just like the various opinions on tire pressure. I was just curious how the service history of the ISX (15) compared to the ISL.