Maybe my world is a bit different, but on 3 different TT's, I used the factory installed tires until one of them showed signs of fatigue. First trailer, never changed them. Originals tires traded with the camper. Second camper, I ran the originals 4 years. They started showing signs of dry rot. Third camper, two years on the road when one tire tread was wearing very uneven. But it took 2 years and thousands of miles too. Swapped out all 4 at the same time, even though the other 3 looked good.
The choice is yours if you want to swap them out or not. If you keep them inflated to the max, inspect them VERY often, and at the first indication of anything unusual, then swap out all 4 at the same time, why not give them a few miles first? BUT! To each his own.
About the tire rating, others above stated it quite well with the tongue weights and such.