This is a no brainer and a smart rule, especially in Arizona. Yes, a 50amp coach will certainly work with 30 amps using an adapter, but why would you UNNECESSARILLY force that issue. In Arizona, known for it's heat, you're going to have people who will push their 30 amp rigs to the max by trying to run both AC's. This can lead to tripping the breakers multiple times, eventually ruining them. Even though there is an electrical code that many will quote, it doesn't mean the campground has brand new wiring that can withstand the abuse.