I have owned three TMs - a 2002, a 2006, and now a 2020. We have always experienced dust infiltration when driving on unpaved rural roads. But it sounds like you are describing something much more pervasive.
My theory - and it is only a theory - is this. As you know, when the TM is collapsed for travel, the front and rear shells are not rigidly attached to the frame. They can move up and down a bit. As they move, they work as an air pump. When they move down, air is expelled from the interior spaces, and when they move back up, air is sucked inside. It seems reasonable to me that this pumping action would bring in some dust and dirt. To address this, you might try to limit the amount of up and down motion by adjusting the center latches and the corner latches. Don't tighten them down too much, as the corner latches especially are prone to pulling out. But they shouldn't be floppy, either.
While I believe that this pumping action could bring in dust and dirt from the roadway, it seems a bit harder to believe that it could bring in leaves and twigs. I'm guessing that the bag seals are not adequately sealing. Note that if the center latches are too loose, the roof seals may gap open, bringing in leaves and twigs from the roof. In addition to tightening the center latches, you might try sweeping the roof before you start each day's trip.
In addition to the above, on all three TMs I have noticed that the biggest area of dust and grit infiltration (but no leaves or twigs) is inside the floor-level cabinets around the oven. This seemed to be because the black plastic wheel well liner wasn't adequately sealed to the floor. Application of some silicone sealer all around the wheel well liner helped, but did not eliminate this problem.
Bill
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