I can only speak of road & hybrid bikes w any first hand knowledge. in general, it's usually a trial & error kind of thing. safe to start higher & let a little air out if the ride feels harsh or if you're sinking in soft, sandy dirt. so use max for the road then let a cpl lbs out for the rougher dirt. they hold less air than tires I'm used to, so when you let the air out go slow. counting to 1 might be all you need to do from max. if they aren't bulging & you feel they are still too hard & narrow, count to 1 again. but that would probably be it. again just guessing here. when you hit the pavement again you're gonna wanna pump them back up. but that's a lot of tires to be pumping even w CO2

if the trailer is light & bouncing a lot, it would probably be more stable with slightly softer pressures