Need to replace tires on my 38' fifthwheel that weighs 18,000 lbs Tires that came with rig are five ply I got a great deal on 12 ply tires Any reason for not upgrading to 12 ply tires?
Ignore "ply" ratings. At one time it really had meaning. Not much any more. The best thing is to look at actual load rating (in pounds) on the side of the tire.
Also remember you can use LT (light truck) tires in place of ST (specialty) as long as they meet the load rating. Usually more choices in LT tires.