How to find proper tire pressure for tow vehicle?

Tire pressure chart shown will get you there.
With 285s if it's a gasser I'd be at 45-50 F and 35-45R driving empty.
Diesel I'd maybe bump up the front another 5 lbs.

Load er up and raise the rear pressure accordingly. Maybe the front of you want it to ride stiffer or have transferred or added more weight to the front axle.