EDM, even if the tire was made specifically to fit a Belchfire 37 footer... (Made up model) they can not predict how many pounds of food, cloths and even PEOPLE will be inside the rig as it goes bouncing down the highway.. So they can not predict the proper tire pressure.
THUS, you MUST put it on a scale and get your 4 corner weights (Procedure elswhere in forum, about a thousand elsewheres) that is the ONLY way to figure out the proper PSI.
And apologies to the cartoonist who has his characters, from time to time, visiting Belchfire Motors for his new car.