On my 2001 Itasca, the placard says to use 245/70 19.5 F (12 ply) tires and 70 psi. How messed up is that? Itasca put G (14 ply) tires on the coach from the factory. They have no explanation as to why. For my weight, I needed approx 85-95 psi in each tire. Best advice is to weigh the coach, know your max rim psi, and air up according to the inflation tables plus 5-10 psi. For a 20,500 chassis, I would think "G" 14 ply tires. Just my opinion, stay away from the G670RV 19.5 tire. I personally don't think it is a good tire.
To address your question, I don't know. Cost? ignorance?
