As far as engine braking in the mountains, there isn't much for sure. Our 2006 chassied Chevy class B with the 6.0 gas engine was wonderful going down the mountains, just not up. The turboed diesel engine w/o a Jake brake in the Sprinter is the opposite, great going up but hard on the brakes coming down.
I noticed this first with our 2009 class C Sprinter we had and now again with our B, 2013 chassis when we had it in the Arkansas hills. In fact the 2013 seems to have even less engine braking than the 2008. Both are 3500 dually chassis