P rated tires off road are like wearing flip flops instead of hiking boots on a backpacking trip, they work but the chances of damaging your feet are a high probability...
I have a 97 F350 4x4 7.3 crewcab with BFG 285-75-16 and regular driving without load I run 50 to mabey 55 PSI front and rear.....I go more PSI if im hauling firewood or something heavy....
I personally wouldn't put "P" rated tires on a landcruiser especially loaded with camping gear and used on dirt roads...its kinda like putting an 3000lbs ATV winch on your cruiser and hoping it will be enough...just my thoughts..