I used to have the Cayenne Turbo, but replaced it with a BMW X5M.
Both are quite similar, and both would make a good tow vehicle for a 20-25' boat, but I wouldn't choose either for a travel trailer or any other high-profile trailer.
I have a variety of trailers that I tow - two different 25' wakeboard boats, a 27' power boat, a 22' toy hauler, a 26' enclosed cargo trailer, and a 24' flatbed open car trailer. All of them are in a similar weight range when loaded (around 7000 lbs) but the dynamics of towing are VERY different.
I would be quite comfortable towing any of the boats with the BMW or Porsche, mainly because of the low center of gravity, axles offset to the rear of the trailer, etc... But the cargo trailers and toy haulers are entirely different - the axles are near the middle and create a pivot point, and the large sides are much more susceptible to cross winds. The smaller SUVs wouldn't be my first choice for those. For the larger trailers I use my full-size trucks - Yukon XL Denali and a Duramax Dually.