First, do you already have an OEM westfalia hitch, with proper electronics? If not, very hard to find, and I would not trust an aftermarket hitch [given the necessary installation into the unibody construction] to tow even a small/light travel trailer. See the long DIY hitch install thread here for lots of info. Second, do you have V8 E53, and the fancy air adjusting rear suspension? The 3.0L, with automatic, is only rated to 5k, and the manual 3.0L is tested pulling over 4k lbs or so [I've hauled maybe 4800-5000 of hay and trailer, and it really tests the 3.0L in even moderate hills]. Third, these are short wheel-based vehicles [111"]. The sail area of a travel trailer means, with a short wheelbase and the relatively heavy tongue weight of a travel trailer, you will likely need a good antisway/weight distribution hitch added to the trailer [Blue Ox, Andersen, etc.]. Check out the good rv.net forum for info, and the airstream owners' forum, which has lots of smart posters. You will also need the RF Tekonsha Prodigy brake controller, as you don't want to mess with the wiring in an E53 to hard wire a controller. Given the stuff people normally carry in trailers, I would be reluctant suggesting anything heavier than 4,000 empty trailer weight as your goal. I've hauled big horse trailers behind Ford diesel pickups, and am about to buy a light 2 horse tag along we can pull behind my E53 and the CEO's diesel Benz ML350. It weights 2350 lbs; loaded with horses and tack, that puts us around 5k lbs, as much as I would haul even with the 6k rating. I'll see whether I need to install a weight distribution/antisway hitch - I expect I will. In short, E53s are not particularly well-suited to hauling their 'rated' weight, though they are fine for lighter loads and, with a proper WD hitch, brake controller, and particularly V8 power, are likely fine for occasional hauling of boat, horse trailer, travel trailer, etc. with a gross loaded weight around 5k or so. If I towed a lot at that weight, I would also consider investing in a tranny temp gauge and the possibility of a tranny cooler with an auto tranny. In short, you have lots to consider beside trailer weight to consider, but I suggest a 4k max empty weight is a start [and trailer salesmen never seem to know what the trailer actually weights ......]. Good luck!

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2006 X5 3.0 6-spd w/Evo UUC ssk
sport/premium pkgs
born Valentine's Day, 2006.