My experience on the X5 isn't with an enclosed trailer, but I have towed close to 5000 lbs and it is remarkably stable. Much more so than previous Expeditions, Explorers, and heavy duty pickups with similar and larger trailers. I have also driven commercial vehicles, with trailers, and I continue to be impressed by the X5's towing prowess.

Tell your friends that a key design feature is the distance from the rear axle centreline to the hitch ball. It is much shorter on the X5 than on many larger vehicles, hence there is less of a 'lever' for the trailer to wag the car. This is exactly why 5th wheel trailers are more stable than traditional hitches. The inherent stability and handling of the X5 helps as well.

You mention a good hitch, and trailer brakes, both good points, but I also think that prudent driving practices play a part. Watch your following distances, slow down in heavy cross winds, and anticipate traffic as much as possible.

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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