Tow Dolly Tires D vs. R what's the difference?

I looked up GY Marathon, and the 13" and 14" are available in Load Range C only. Starting at the 15" sizes, Load Ranges C and D are available. Somewhere on the sidewall your new Marathons should say something like "Max Load 1760 pounds at 50 PSI." If the tires you're replacing have a similar rating, you should be fine.

Ok, so there's minor difference's to the 2 types. The D has cross woven cords and is considered a durable tire that withstands uneven surfaces better than a R rating,sidewalls are stronger with a lower speed rating. there designed to withstand the UV rays also.
The R tire is acceptable to use in place of a D rated tire. Some of the benefits are a cooler running tire more design for highway use, with cords that are parallel instead of a woven design, improves gas mileage and are typically less expensive than a D tire .
So I bought Goodyear marathon ST20575R 14s and expect no difference between the old 20575D14s.

Trailer and Light Truck tires should bear "ST" for Special Trailer or "LT" for Light Truck. If there's "P" its Passenger Tire and NOT designed to operate at the top end of its load rating. "P" does not bear a "Load Range" designation, but both "ST" and "LT" tires will. To bear this out, I looked at the radial tires on our boat trailer. You have a 14" wheels, so if I adapt what the sidewall of my Radial Trailer Tires said, you need labeling like:

ST 205 75 R 14 D Or perhaps the ST205/75R14 in one place and Load Range D somewhere else on the sidewall.

Many use "LT" tires on trailers in place of "ST" (but not the reverse) and that seems OK. Using "P" series on a dolly is not acceptable. And Load Range C (max load rating at 50-psi) is also probably not acceptable. I think dolly tires should be Load Range D, carrying their max rated weight (shown elsewhere on the sidewall) at 65-psi.

A dolly came with our coach when we bought it used. Tires were 13" but only from memory the Load Range was D.

If the tires you bought include that "D" associated with "Load Range" I think you're OK.

Just Do Not, Don't, No way, put Passenger Car tires on that Dolly!

I don't know much about tires.

However, I did remember seeing this info about Radial vs. Diagonal Tires.

I assume that is what you are talking about. But, I've been wrong before.