Unless the area where you are is really dusty, I would be looking for a leaky window, ceiling vent that isn't totally closed/doesn't seal, etc. ... or tell the kids to keep the door shut!!
I recall a few months ago someone having a huge amount of dust, particularly in the bedroom and is drawer in a dresser, on a new coach (different brand than yours). Inspection at night using lights revealed a mfg defect where road dust was coming in the bottom of the coach while driving. ...certainly not a likely problem, but if you can't explain the level of dust any other way anything is possible!!
Later note ... I found the link in your sig line didn't work. When I tried some alternatives I found the following location: http://web.mac.com/wahlsacross...America/Welcome.html