According to the brochure goducks10 posted, your rig was placarded for =C= tires. If you've got D tires on it, you're actually above the factory recommendation. Personally, I see no reason to go any heavier than a D, though an E would be the next step. As donn said, the Goodyear Endurance seems to be doing quite well, though I've had good luck with the late model Carlisle tires and have no problems in recommending them. They're also cheaper than the Endurance. The old go-to tire, Maxxis, is being left in the dust by these newer tires, which are not only enjoying a better rep, but are speed rated to at least 10 mph over the Maxxis, which are still stuck at 65 mph. While I don't tow > 65, I figure that this means there's more safety margin while towing at my usual 62-63 mph tow speed; less heat build-up.
Lyle