Your trailer should have 225 75R15 LRE tires (not 215 75R15).
Trailer King is one of the brands that is "affectionately" referred to as China Bombs. There are others, typically "off brand ST tires"
Most have relied on Maxxis, Carlisle Radial Trail HD and Goodyear Endurance.
Right now, WalMart has the best price I've seen on Carlisle ST tires, $82.19 each.
https://www.walmart.com/ip/Carlisle-...-tire/55012166
Goodyear Endurance is the "new kid on the block" with about a 3 year history since introduction. So far, it seems to be an excellent choice, although it is about $50-75 more (per tire) than Carlisle.
Maxxis, from what I've seen, hasn't "kept up with the competition" with their M8808 tire line. The speed rating remains at 65MPH while Carlisle and Goodyear are 75 or 81 MPH rated.
ALL (regardless of brand) ST tires should be replaced at abour 5 years of age, no matter how much tread is remaining. ST tires typically "age out" before they "wear out"....
There are some people who have reported good service from Trailer King tires, some that have reported damage in the $5000-$10000 range from "self destructing TK tires"... It's up to you whether you decide to keep them or replace them. I'd suppose the biggest concern is "previous use conditions" ranging from towing with low air pressure, sitting for months on a dealership lot and stress from "jockeying trailers in tight turns on the dealership lot. With OEM tires, you have no way of knowing how they were handled before you bought the trailer. With tires installed "while you watch" and that "remain in your custody" will have been cared for by you, so you'll "know for sure" what condition they are in....