Empty, you've got about 5,000 lbs on the front axle (2,500 lbs/tire) and about 3,000 lbs on the rear (1,500 lbs/tire). The D range tires you suggested will work just fine. Tire wise, they would put your carrying capacity to almost 7k lbs on the rear axle (almost 4k lbs of payload). There's no way you'll exceed that with a 7,000 lbs bumper-pull trailer.
If I was pulling long distances in the middle of summer heat and I was close to my tire's capacity, I'd go for the higher rating. Since you're not anywhere near any of those (especially the tire capacity), you've got nothing to worry about.
The letter rating is relative. The weight rating is what matters.