May I suggest that when you are moving through the air, a partial vacuum is created back there and that's what causes the dust to swirl back there in a large horizontally oriented eddy. It looks like a hydraulic in a small water fall, that's why kayaks get stuck in hydraulics. Race cars with spoilers make bigger eddies and it forces the car down on the track.
Air essentially does the same thing in the back of your motorhome and it isn't related to static electricity. The air goes over the roof and when it comes over the end of the cap it begins a downward roll and it continues until you're stopped.
My motorhome get a lot of dirt on the back of it and every shelf like space has an ample deposit of sand and or debris. 1st order of business when I get into a site is to blast away some of the dirt if I can get away with using a bit of water.
This process gets amplified by 100% in rain when it combines with road dirt and leaves an immense mess all over the rear of the coach.
Have you ever seen a car or SUV with a rear deflector? It does keep the back of the vehicle a lot cleaner because it forces a high speed jet of air directly downward over the rear of the vehicle blowing off deposits that would be swirling around in the partial vacuum.
....and that's all I've got to say about that!