The poster with the 28' Airstream at 8300 lbs was withidl. He had lots of posts re towing if you search. Withidl hasn't been on since mid 2007. He wrote about crossing the continental divide several times with the trailer. He had Dinan throttle body, engine software, and transmission software, but no changes to the factory radiator or transmission cooler. I don't see the point in upgrading either one, but I would ensure the coolant is in good condition. I don't think the cooling systems on the V8s are sensitive if maintained, as much as the engines are not tolerant of cooling system failures and subsequent overheating. I don't think of it as a capacity issue as much as I do a consequential damage issue.

One of the long term posters with a tall vehicle trailer was hayaku, also with lots of posts. He has been around as recently as earlier this year. He ran his car trailer at about 7000-7500 lbs, and wrote about pulling it in California heat in the mountains, and on the Grapevine.

Lots of posts from others towing similar trailers. This is going back three to four years, when people were learning about E53 towing limits.

Both withidl and hayaku used load-distributing hitches. Good idea. I wouldn't try and put the weight in the X5 instead of the trailer if you are set on taking it with you anyway, since it is the combined weight that impacts the vehicle. All you are doing by moving the load into the truck is compromising the handling.

I wouldn't worry much about upgrading the brakes, but I would maintain them, and the trailer brakes, religiously.

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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