There may be diesel exhaust leakage coming from under the bed area, which sometimes is not sealed as well as it should be. It can get sucked back into the coach when driving down the road, especially with windows or vents open. Of course, the exhaust is all supposed to be going out the pipe anyway, so maybe a problem there?
But if the "soot" is just powder and not oily, it probably isn't diesel exhaust.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL