If the Marathons were OEM they could be made in the USA back then. They were moved to Canada for a couple of years before being shipped to china.
A 4000 lb dry weight = 1000 lbs per tire requirement.
A 6000 GVWR = 1500 lbs per tire requirement.
Actual weight on the tires is 6000 lbs less 600-800 lbs for the hitch carried by the tow vehicle. Higher rated tires won't help with a overloaded trailer.
The Kumho 857 have a good reputation across the trailer world including RVs.
The Maxxis U-168 commercial grade LT tire in several 14" sizes are also popular where they are easily available.
