A brief comparison.
The GM 2.8L 4 cyl diesel produces 181 hp and 369 lb/ft of torque
The Mercedes 2.1L 4 cyl diesel used in the Sprinters (and yes, motorhomes have been built with this engine) produces 161 hp and 265 lb/ft of torque
The Mercedes 3.0L V6 diesel used in the Sprinters produces 188 hp and 325 lb/ft of torque.
So while the Sprinter powered motorhomes are somewhat underpowered, a GM Savannah cutaway cab/chassis with the diesel would have similar performance and probably do as well.
The auto makers are intent on going to smaller engines, and the market suffers the consequences. This is why some manufacturers are beginning to build large C's and super C's on the F450/550 and Dodge 4500/5500 chassis's. People want something rock solid and with performance that will get them there, despite the terrain.
Charles