We had the same problem when we were traveling out in Utah. Really dusty there! We took a large towel and placed it on the floor by the doors, we damped it first. It did a pretty good job.
You could also put one of those "skirt" underneath by your bumper, like some of the larger RV'S do.
Bobbi & Harold Bergstrom
EASTCOAST AMBASSADORS FOR PLEASURE-WAY
PLEASURE-WAY RV CLUB EAST PRES. & VICE
2003 FORD EXCEL-RD (#3)
HAVE PLEASURE-WAY, WILL TRAVEL!