Tow vehicle pressures should be set according the tire pressure label on your door jamb. Those pressures are typically listed for laden and unladen. You will need to determine how much payload you are carrying (which includes the trailer tongue weight) and the effects of weight distributing hitches (if any).

You should also confirm your actual real world tongue weight. Published specs by trailer manufacturers are notorious. Load your trailer as usual, and weigh it to know for sure. Loading the trailer can have a significant impact. Also, weigh your vehicle on a highway scale, loaded, to ensure you aren't exceeding front or rear axle limitations.

Your air suspension has the potential to complicate setting up weight distributing hitches. Just because the vehicle is level doesn't mean it is balanced.

I towed up to 4500 lbs at 32 psi front and rear in my E53. If I was heavily loaded in the rear, I occasionally increased rear tire pressures to 35, and reset them after the trip.

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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