We had a similar problem on our Ď06 Phaeton and I think the replacement part was called an air dam or something similar.
It's a piece of rigid plastic on the leading side of the door and goes from top to bottom. It sticks out perpendicular to the coach about 1Ē. Ours was broken and missing in some places, replacement was easy and fixed our problem.
Denny, Connie & Shadow (former barn cat made good)
2006 Tiffin Phaeton, Cat C7, Freightliner Chassis