Jesseannie - I'm an AZ resident too. First thing, check the manufacturing date on the tires to see how old they are. You'll find a code with four digits. For example, if you see 3911, it means the tires were manufactured in the 39th week of 2011.
My assumption is that your tires are older than five years. If so, please do yourself a favor and replace all of them. This includes the spare if it's over five years too.
Even though the tires that had the most sun exposure show the serious damage, I'll bet you that the others are also showing signs of upcoming failure. You certainly don't want to have a tire failure while on the road. It can be very expensive, do damage to your trailer, and put you and others on the road at risk.
Jim