tires

Actually they ride no different than the 16's I replaced. I ran them at 80 psi, per their chart, which gave me a capacity of 4500 pounds, all I needed was 3,500. First time out with them, my fuel mileage was down 1.5 mpg, and they ran hot!. I aired them up to the 125 psi for our next trip, temps where in the 90's, tires ran cooler than the truck by 30*, fuel mileage came back up.

I run the speed limit, and yes that is 70 mph. I am running at half the capacity of the tires and it is running cooler than the 16's I had by 30*+. Heat is what kills tires and causes blow outs

Good catch and good for you for not blindly following some inexperienced tire dealers and even tire mfg advise of derating a tires pressure on a trailer.

And you were wise enough to understand how to fix the drop in mpg and hot tire problem.
Great post.

I decided to move from Load Range C to Load Range D tires, same size tire. I noticed the new Load Range D tires are a little heavier that the Load Range C tires that had been on my 5th Wheel. Should I be concerned about moving to the Load Range D tires?

You have the same scenario however at a much much smaller scale of 50 psi v 65 psi. It all depends on all your tire/axle wheel weight numbers .....but generally going from a C load range to a same size D load range tire makes a good upgrade especially if the wheels can take 65 psi.