No more damage than if you drove the same 15 miles without a trailer. I wouldn't worry about accelerating the wear on the seals, they took tens of thousands of miles (since new) to get to the state they are in.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue