Camper can suddenly shakes when parked

I have a 2003 Chinook Concourse. Yesterday when I was in the cabin area, it started noticeably shaking when parked and not running. Whether it was the wind or a truck driving by, it shook a little bit. I moved to a different location thinking maybe I was on uneven ground, but it still shakes. This has never happened before...it always stayed pretty still. Any idea what it could it be?