On dusty roads I use my fantastic fan on low in to create the pressure. Just have to remember to come to a stop slowly so the dust doesn't catch you. To solve this poblem, could easily make a simple HEPA rated filter to take care of the dust befoe it gets pulled in.
Or for simply method, make a hepa filter with a simple 12V computer fan to create the pressure all mounted to a piece of plexiglass or wood that fits into the pass through window or some place else. This will probably be my solution for the next camper when I start traveling on more dirt roads where other traffic is likely.