Floorplan vs engine: is Cummins 300ISB 'enough' (39' coach)?

She wins. Years ago when we had a 38ft motorhome with a 454 engine pulling a station wagon I could only make it up the Grapevine (I-5 Bakersfield to Los Angeles) at 25 to 35 mph. It added a half hour to trips in that direction. We made it, and had wonderful times. Now I have the Cummins 340hp and pull a 3700lb car. 45mph is really tops over that grade, but it is an easy pull. I intend to get a larger toad shortly, closer to 4700lb, and I don't anticipate any problems. Enjoy and Happy Trails.