myowneq from Louisiana:
You are correct that the pressure will increase if you air up before putting a significant load on the rear axle like a truck camper. I load the camper and then air up my firestone rite rites. Measure the fender wells or rear bumper of the camper on each side to determine how much air in each bag to have truck level side to side and front to rear. I have overload leaf springs and I have found the best ride is with 1 end of each overload on each side engaged on the stops. On my setup I run ~35lbs on the drivers side and 65lbs on the passenger side. Trial and error will help you! Mine are 15 years old and still work great. Check the mounting bolts to the frame as I've had these work loose.