The Endurance replaces the old Marathons.
Tires cupping in the center of the tread can indicate a brake lock up event (flat spotting). My 10k car hauler had one brake that flat spotted a 16" E tire in several places around the tire which caused a bad vibration (empty trailer) to the truck. I use it as the spare know.
OEM 6 ply would be a C load range. A load D would be a better choice. At 1.2 million miles pulling trailers with 14"/15"/16" and 17.5" tires I found out the hard way a trailer can be over tired.
Thats why experts recommend a replacement tire with not over 15- 20 percent reserve capacity above the trailers axle ratings. That way the tire can be ran at max sidewall pressures for a cooler running and longer lasting tire.
I'm not a fan of any ST tire because of my past experience with them however the Endurance....Carlisle HD....and the Provider ST tires seem to be the better choices when we look around the net at the many websites that use rv and non rv trailers.
I have the Provider ST235/80-16 E on the 10k car hauler (5200 lb axles) I mentioned. Their DOT date code is 2610 and 2510 or 9 years old. The tread wear bars are starting to show so I just had Discount Tire mount four more. I'm sold on the Providers.
Check them out.