My understanding is that the sidewalls of tires develop cracks from sun and weather while the RV sits parked and undriven for most of the time. It helps somewhat to use tire covers. If sidewall cracks are more than 1/16 deep it is wise to replace tires. The DOT date codes embossed on tires tell what year and month tires were manufactured. Make sure that "new" tires are not old stock.
Tip, if you hear/feel thumping from a tire while driving, it may be a bulge where tread is separating and tire is about to come apart or blow out. Don't risk driving on old tires. Buy as highly rated tires as you can, it's worth it!!
Tip # 2, have brake fluid flushed and replace front flexible brake lines while having brake system inspected and serviced.