Not sure exactly what you're discussing. Is it the smog inspection sticker needed to register the motorhome or is it the CARB declaration?
If it's the emissions/smog sticker, you need to get that from the station prior to registration unless it was already registered in CA with a current valid emissions/smog sticker. Since they want one, your MH probably never had one.
If it's the CARB declaration, you can get that from the CA Air Resource Board. They issue the declarations based on engines. As an example, here's a sample of one for the 2006 version of your engine:
2006 CumminsIs this what Cummin sent you?