It is actually better for the tire to be rolling down the road than sitting idle. While in use, the tire releases lubricants that are beneficial to tire life. Frequent use of your trailer tires can also help prevent flat spots from forming.
Tires that are inflated to less than the recommended inflation level of the GVWR of the trailer is exceeded, the load carrying capacity of the tire can be dramatically affected. Tires are inflated to more than the recommended inflation level can cause handling problems in the tow vehicle/trailer combination. You want to check the cold tire inflation pressures at least once a week for proper inflation levels.
Wheel and tire manufacturers recommend adjusting the air pressure to the trailer manufacturer's recommended cold inflation pressure, in pounds per square inch (PSI) stated in the vehicle's Federal Certification Label or Tire Placard when the trailer is loaded to its gross vehicle weight rating (GVWR).
Tires can lose 1 to 3 PSI per month. This is because air molecules under pressure weave their way from the inside of the tire - through the rubber - to the outside. A drop in tire pressure can cause the tire to become overloaded, leading to excessive heat build up. If a tire is under-inflated, even for a short period, the tire could suffer internal damage.
Statistically, the average life of a trailer tire under normal use and maintenance conditions is 5 years. After 3 years, you should consider replacing your tires with new ones, even if the tires have adequate tread depth. Some experts say that after 5 years trailer tires are considered worn out even if they've had minimal or no use. This may not apply in every case, so it's best to have your tires inspected by a tire supplier to determine if they should be replaced.
Another thing to consider is that high speed towing in hot conditions degrades trailer tires significantly. As heat builds up during driving, the tire's internal structure starts to break down, compromising the strength of the tire. Be sure to drive at moderate speeds to extend the life of your tires.
If you are storing your trailer for an extended period, make sure the tires are fully inflated to the maximum rated pressure and that you store them in a cool, dry place, such as a garage. Use tire covers to protect the trailer tires from the harsh effects of the sun.
This, of course, is not everything you need to know about tire safety, but it's a start. You can always refer to your owner's manual for more information.
Taking good care of your tires will not only keep you and those around you safer, but will also save you money over time. It makes a difference.