Your trailer's ability to perform its function relies entirely on the health of its tires. Poor tire condition impedes your trailer's ability to haul things, and it also creates a dangerous situation. To prevent dangerous situations and improve the performance of your trailer, it is critical that you perform regular tire maintenance.
Tire function is a combination of two factors: their ability to hold pressure and their tread. As such, the two main focuses of your maintenance are always your tires' air pressure and their wear. As with all tires, trailer tires are prone to developing slow leaks from use over time. However, trailers are also far more likely to see sporadic use than vehicle tires, and so they also lose air from disuse. Before each time you use your trailer, check the pressure of each tire, and add air as necessary. Proper air pressure is important as low pressure potentially leads to blowouts, swaying of the trailer and unnecessary tread wear.
Though abnormal tread wear results from under-inflated tires, you must also check for the accumulation of normal tread wear. Due to the heavy loads they bear, trailer tires wear more quickly than vehicle tires. If you use tires without enough tread, you risk several dangerous situations, including blowouts and losing traction in poor weather.
If your tires are not holding air properly or have excessive tread wear, you must replace them. It is usually a good idea to replace all your trailer's tires at once. Uneven levels of wear across your tires sometimes leads to swaying. Different tires compensate for sway in different ways, and you do not want to mix tire types unknowingly.