OP wrote:
How many miles should you expect to get out of a set of tires on my 13k trailer? Currently I figure I have gotten about maybe 15k. traveling across the country from coast to coast and local outings. Never have rotated them which I never thought of doing but do on my truck and cars.
Its recommended ST tires be changed every 3-5 years. Trailer tires are next to each other in the center of a big heavy box and all the side scrubbing around corners wear evenly unless a bent axle or other suspension issues.
I've worn out lots of 15"/16"tires on my trailers in my business. Never rotated one nor did they need to be unless something got bent or worn.
I know brand and construction of tire is a biggie. I have on some junk tires {which I think they are} Called "Gladiator" ST 225/R15. Bought new at L.S. a big tire dealer/shop up here in the Northwest. Welcome any input, thanks in advance....
As you suspect the ST Gladiators isn't one of the better tires for a heavy road trailer. Their popular around here with with feed stores and small town tire dealers that offer a low cost tire.
Many folks come on the forum and complain about their ST tires (all brands) and are looking for the best replacement. Complaints range from sudden blow outs with lots trailer damage to tread loss or separation issues.
The most popular tire upgrade for a 13000 lb trailer is upgrading to 16" wheels and LT E tires. Your tires may be a ST225/75-15 D load range at 2540 lbs capacity and are 28.3" tall.
The LT225/75-16 E is 29.2" tall and has a 2680 lb rating.
If you have room the LT235/85-16 E at 3042 lbs at 31.5" tall will give you 12168 lb of capacity.