Just keep in mind that what pulls the dust in the back is the vacuum created against the back doors by the van's forward motion. Best thing I've found to keep the dust out is to run the fan motor on high and create positive pressure inside.
Just got back from a trip in which I drove over 200 miles on dirt. This year having been a severe drought here in Nevada, the roads are disintegrating into what the locals call "bug dust" - huge bowls and strips of fine silt. My van was so covered that before coming home I had to visit the nearest car wash. But the inside stayed clean.
Layer of "bug dust" coated on the back of my quad. Can't even tell what the original color of the travel case was!