Some class C/B+ motorhomes are quieter than others. Our Winnebago Aspect is very solid and we can listen to the radio and talk comfortably. We did have to find some of the noise makers. Like the tray in the microwave, and silverware, dishes in the cupboard. Lining the shelves with that waffle padding and putting some around the silverware made a big difference. We had an annoying squeak that took some time to find (cover to the converter).
Now the only issue I have is something inside the microwave case rattles and when we are hit bumps the E450 springs are pretty stiff.
Don't believe that any of the Class C/B+ motorhomes will be quiet like riding in a car. The more expensive units should be quieter...don't know how much. The E350 chassis rides softer and quieter than our E450 unit.
We don't mind the engine noise and I can usually keep it below 3000 rpm except on some of the steeper hills.
Good luck finding an acceptable motorhome.