The biggest factor in how loud it is inside from road noise is tires.
You'll find that the quietest set of tires will lower the interior noise
waaay more than any amount of insulation or aftermarket sound deading
material you could ever stuff into your van.  Some tires are simply VERY
NOISY!  Others are ---much quieter---.  If reducing road noise is your high
priority, perhaps you should ask around the list to see which brands get the
"Shhhsh-Award" for the mostest quiet of all...
  Even in my Luxo-928 P-car with 3800lbs of sound-reducing mass (when stock)
if I put on Kuhmos or BFGs, I could hardly hear the sound system..Pirellis
or Michelins...very quiet indeed..huge difference.
  Don Hanson