I was amazed how quiet in was on the initial start up in the bedroom...ours coach is so much noisier in the bedroom with the motor (C7) running. As an aside.....did you notice that when your service brakes are applied, the drivers side reverse light comes on? Love your motorhome!
If you are going to straight pipe the exhaust aka, no back pressure (which is ideal for a diesel), you need to make the intake less restricted as well.
it could be that the governor is set for that 68-69mph does it go faster on flat land or is that the max speed no matter what? if its governed at 69 u can turn it up higher or have ur local diesel shop do it