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
2016 Ford F350 Turbo Diesel SRW 4x4
2017 Grand Design Reflection 297RSTS
2013 Ford F350 Turbo Diesel SRW 4x4 (Destroyed by fire - 8/29/16)
2014 Grand Design Reflection 337RLS (Destroyed by fire - 8/29/16)