Regarding the older DP's, my experience has been that they are normally very quiet but, under certain wind conditions there is a bit of wind noise. However the mere fact that they are normally quiet a little bit of external noise is more noticeable when it's not competing with the engine noise level. The tinkling of the wine glasses and occasional shades rattling are more noticeable than the wind noise.
Road noise is quite often the tire thread pattern and road surface. I just replaced my front Toyos that had a M/S open thread with ribbed thread Hankooks and the difference was amazing.
Here again, even with the DP's the external noise will probably differ depending on the coach but still should be comfortably quiet.
Good luck in you quest.
