If the tires on your towing trailer seem to be worn down unevenly, this is often caused by a misalignment. As with your car, when tires on a trailer are not aligned properly, they will pull or drag to one side and, in turn, the tread will wear down unevenly. This is not only damaging to the tires but can also be dangerous when you're on the road, as it makes it difficult to control your trailer.
Note how you can check the alignment of the tires and trailer yourself; for this, you'll need a long, straight rod of some sort, such as an oversized metal bracket or even one side to a bed frame. You may also need a friend to help mark some measurements.
Hold the metal bracket against the two tires on the right of the trailer, then measure the gap between the back end of the bracket and the back of the trailer. Next, measure that gap between the front of the metal bracket and the front of the trailer.
If those measurements are not the same, your trailer has a bent axle. If the axle is bent, the tires will generally push outward, also pushing the metal bracket away from the trailer. If the gap is larger in front than the back, the front axle is bent, and vice versa for the back axle.
The spindle is the rim of a tire and when one is bent, that tire itself will be out of alignment. To note if a spindle is bent, hold the metal bracket against the tires on one side and check if there is a gap between the bracket and one tire. If so, this usually means the spindle of that tire is bent and it's keeping the tire from sitting straight. This tire will usually have uneven wear and tread, but simply changing the tire itself without having the spindle repaired will mean that your new tire will also become worn down unevenly, so have that spindle fixed or replaced as needed.
Frame out of alignment
To note if the frame itself is out of alignment, use your metal bracket or a measuring tape and measure the trailer from the centre of the front to the outer left corner in the back. Next, note the same measurement, from the centre of the front, to the outer right corner in the back. If the measurements are different, the trailer itself is bent and needs repair so the tires all rotate smoothly and evenly.
For help with these measurements and with fixing your trailer, contact a trailer repair professional.