If you buy an Endurance ST, you will take the load range they offer in that size. GY only makes one load range in a given size, and it is the "upsize".
GY makes a total of 7 different part number Endurance ST tires, two sizes in 14 inch, two sizes in 15 inch and three sizes in 16 inch.
https://www.goodyearrvtires.com/tire-selector.aspxST205/75R14 D
ST215/75R14 D
ST205/75R15 D
ST225/75R15 E
ST235/80R16 E
ST235/85R16 E
ST255/85R16 E
If you want LT tires, there are no true LT tires made that I have been able to find, in 15 or 14 inch. 98% of the trailers out there use 13/14/15 inch, so LT is not an option. On some trailers its possible to go to a 16 inch rim and install an LT tire, Airstream does this as an option from the factory on some models.
Charles