When we ordered our 2008 MH (in June 2007) the ISL was a $6,000 adder over the ISC. At the time Newmar had a few pre-emission, 2006 spec ISLs left. If we agreed to take one of these pre-emission engines they'd do the ISL upgrade at no additional cost.
Honestly that, and the addition of Comfort Drive, were the 2 biggest reasons we traded. 10 years later I'm pretty happy we made the move when we did. We were a bit concerned with the newer emission regulations and the effects they may have. Reading through this thread, I never realized it wasn't a smooth transition - until now.