The saying is “Fifth wheel tires don’t wear out, they age out.” With that said I have Sailuns that are only 3yrs old that I’m about to replace because of wear; they only have ~37k miles on them :( ...
Rotate the to different locations and run them for a bit longer to verify if it isn't an alignment issue. If your bad tire is worn to far for safety, swap it out with the spare. You need to verify whether or not you have an issue before dropping money on a new set of tires that may get ruined shortly after installing them.