��>>The mass of the vehicle does not change the wind resistance>>

Correct.

<<or the MPG, on the highway.>>

Big No No.

You have to be able to maintain the mass movement.
And this is where the gas mileage gets eaten up, being able to maintain
movement.

On hill or dale, or on the flat.

<<but on flat ground at steady speed the van could be 6 tons and you'd
get the same MPG.>>

Nope.

Lay 12,000 lbs into your Van and see how wonderful the fuel economy is,
on a window pane flat pavement.

I'll run mine empty, at the same exact speed as your going, all the
time.

It takes work to maintain the movement that your Vans going to require
to move and maintain a specific speed that 12,000 lbs. mass will
require.

Better have a vehicle following you, as you'll be thumbing a ride when
the Van runs out of gas.

<<At steady speed you are not accelerating the mass, you are only
pushing the wind out of the way, so mass doesn't matter.>>

Nope.

And how do you get to this magic steady speed ?

Forget the wind, it's only part of the problem you drew.

I think this is a real bad example of what your trying to say.