BFG A/T KO2 Load Range E: what psi for towing?

I wouldn't even be thinking about 50 psi + with those tires and a little trailer.
In my 8000lb (empty) diesel I run about 55F/45R in the summer. Winter may lower all around by 5-10psi from that depending on conditions (snow/ice/how much/ how cold).
I hook up a 6000lb trailer with considerably more tongue weight in the summer with 45-50 psi rear.
I'd start with the same pressure you were running with the P tires before.
quick guess, 35F/30R empty and bump up the rear 5-10 psi with a load.