IS A 10 PLY BEST FOR???

My 12,000# GVWR 5th wheel has D rated tires that I believe are stamped with 2600# a piece tires. The most I've ever had the trailer axles weigh is around 9000# (+/- 20% to the pin/truck = 2000#) If I were loaded to 100% of GVWR it would be close to max on the tires. If/when I replace I'll likely look into E rated for a bit more breathing room. But, I'm not even sure if the rims are rated for more. It is a bit frustrating that the manufacturer's don't build in more of a margin but I also have to remember there is a factor of safety engineered in to the ratings already.