My Class C Majestic is loud and all over the road!

I don't know how accurate a Decibel App is compared to a Decibel Meter.
Either way, the noise is too high in your rig. Can you determine where or what the sound is coming from while traveling?
Is it the things bouncing inside the house, exhaust, wind noise, etc.?
If you can pinpoint the source, maybe you can make some improvements or adjustments.
But, as donn0128 indicated, it may just be the nature of the beast.