We spend a lot of time on dirt roads, and dust infiltration has been our most serious problem. I have caulked and sealed everywhere but dust still gets in, primarily at the bottom of the (rear) entry...
The mud flap will help for stone and gravel but not do much for fine dust . First thing is to get the bottom of the door to seal . One of the m ain culprits is the lower refrigerator vent . The bottom of the frig is usually not seal from the interior . Another thing to check is the range hood vent .