I have this too...I noticed that a lot of the rattle was from the window on the door rattling in the frame. As a quick fix I "jammed" small pieces of wood in the space between the window and the frame. It is much better. The door itself still rattles but the overall noise is better....I have to come up with a more long term solution for the window.
Ours was from the screen door, added extra screws and tightened all the existing ones, problem fixed. I had my daughter ride and see where the sound was coming from!
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Forest River Lexington Grand Touring.
I have found the door to be the most problematic to diagnose. Our screen door rattled against the main door near the handle. Added a rubber bumper like are used on kitchen cabinet doors.