Depends on what you heard. New diesel emissions standards went into effect Jan 1, 2007 and diesel engines manufactured after that date must comply, so yes a 2007 engine is different than a 2006. However, not all 2007 model year diesel motorhomes have 2007 engines.
There is also a phase-in of portions of the new EPA emissions rules (the NOx spec), so engines will continue to evolve through 2010. Here
is a presentation from Cummins Technology about how engines will be changing over the next 3 years.
Cummins also has some new engines for 2008 and I believe they do use a different or improved emissions technology, so at least some 2008 engines will be different than 2007 in that regard. And you can expect that the 2007 emissions technology will be improved to provide better performance and fuel economy as the engineers get more experience with them and refine the designs or come up with better solutions.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL