We have the same tire on ours, about the same age, only our fifth wheel is about 2,700 pounds lighter, loaded and ready to roll. We have 10,000 miles on them and no problems but we will replace them before we make our first trip next spring just to be on the safe side.
I agree the manufacturers could and should make better tires an option. However, many buyers out there will not be knowledgable enough, or don't want to spring for another couple hundred bucks (about 1% on a $25,000 purchase) to order better tires. I imagine that the manufacturers have computed the warranty rate for repairs due to tire failures and have determined they can still save X dollars going with the cheaper tires. Until consumers as a whole demand better tires, as a standard item, we are not likely to see this change.