The importance of reading and understanding the last four digits of the DOT code on tires made since 2000 cannot be overstated. As an example, if the full DOT Code is: DOT U2LL LMLR5107, the last four digits represent the actual date of manufacture. The first two (of those four), indicate the week, the last two, the year. So 5107 means that tire was manufactured during the 51st week of 2007.
Though RV tires may be capable of lasting much longer, all experts agree the effective life of an RV tire is approximately 5-7 years, depending on how much ozone and UV damage it has incurred, along with your particular driving habits. When in doubt, have one of the tires de-mounted and inspected on the inside by a tire expert. A good tire expert will be able to tell if that tire has hit a curb, been punctured and repaired, run under or over inflated, has weakened sidewalls, etc. They all leave their tell-tale indicators on the inside of the tire.
Also, study the great information posted on the official web site of the RV Safety & Education Foundation; www.rvsafety.com.