With our 2016 Vegas, sound and heat from the doghouse doesn't seem to be an issue. The sound we are getting comes through the dash. Only occasionally do I think it might be worth my time and effort to add under dash insulation.
One trick that helps keep the roaring down is to kick the cruise off before the transmission makes that downshift. In some terrains, I don't use the cruise at all. I don't mind if the coach loses speed going up hill, but I do mind when it shifts down to that gear that makes that V10 scream. Of course, on long and/or steep hills it is going to that lower hills anyway.
That's my experience and opinion. Your results will vary!
If all else fails, read the directions!!