Using Old Tires can be Unsafe!
Check the age of your tires and change them if they are getting too old. Most RV and Trailer tires should be changed at around 5 years and vehicle tires 7 years. These are just general rules of thumb that I use. To be sure check with your tires manufacture.
Also, check the DOT code on the tire to see the exact day the tire was produced they may have sat on a shelf for some time before being sold. An old tire can look great and have perfect tread on the outside but be rotted on the inside leading to a blowout. Here is a good link with info regarding tire aging.