Sand blowing through basement a/c duct work?

I think you might find that road sand is being sucked up inside the rear cap. There is a lot of turbulence that creates a vacuum in the rear of a motorhome while moving down the highway. The sand is being sucked up and getting trapped between the roof and ceiling moving forward to the front of your coach.

Same thing happened to me recently but the sand was ending up in my rear bath cabinet drawers. I was lucky as there was enough room for me to get in there and seal the gap. Problem solved.