Original tire size, should be listed on the chassis manufacturers , weight sticker around the drivers seat somewhere and it will give rec-commended inflation.
Tire manufacturers web site will give the specs on the tires, height; Revolutions Per Mile ( when you see RPM , on a tire site , that's the meaning) and load weight rating.
Weight rating is important, my coach left the factory with under rated tires on the front and was recalled, a different tire size was installed on the front axle , because tires that matched the rear weren't available with a high enough weight rating to carry the front axle load.