Reihard,
I don't know how you will do road noise testing without trying to pad
the underside of your Van from the source of the noise.
Your sitting right on top of the major offensive area.
The tires spinning on the road's surface.
The other is the back back side of the Van ( engine, exhaust, tires in
the wheel wells.)
The only proven to work method is dampening the sound with a harmonic,
noise absorbing material, on the top side of the sheet metal, as well as
the bottom side.
The simple to explain way to get this job done is the more padding in
problem area's, the quieter the ride.
I have a friend up in Canada who own's and drives a truck.
He, when the truck was new had the mfg. add the optional winter
insulation kit to the cab of the truck.
On the bottom of the cab floor is glued black, dense, foam rubber.
Under the rugs is more foam rubber of a different density.
Inside the doors is more insulation.
Inside the interior panels is more.
This tractor is like riding in a Rolls Royce.
It's not only drum tight, but almost an ere quiet.
I would suggest to you rather than taking all the time with your DB
meter poking around inside the Van as your driving down the highway
trying to locate the source of the problem, insulate the interior and
the bottom side from the obvious road,& tire noise and your search would
be over.
Also, you might consider replacing the old, shrunk, door and rear hatch
gaskets.
You might think they are making contact to the body just fine, but old
rubber does shrink and will allow air to bypass them.
Have you changed them prior to your DB meter tests?
You should .
It will make a big difference.

RC