I was in the same boat that you are last year and went with Carlisle E rated tires which are good up to 80 psi and I think 81 mph but you won't ever catch me anywhere near that speed. My wheels are 15", 6 lug and are rated for 75 psi and I think my camper has a 10,000 lb. GVWR.
I had also read that ST tires are supposed to be inflated to 100% psi but after some more reading (I can't link you to the exact site) I found that you could run them under 100% but that the load rating would go down.
A good D rated tire would work for you but I just wanted to have a little more assurance that my tires would hold up. Just make sure you get a good tire.