It is a safe route and very popular with RVs of all sizes. However, you will have a steep downhill from Payson. It's very doable but use your gears and get into a lower gear before you even start downhill. It's a gorgeous drive. Enjoy!
If you take I-40 to I-17 you will also have a couple steep downhills so definitely go your alternate route.
The Phoenix area is in a low valley. There's no other way to get to it except by going downhill.
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Full-Timed for 16 Years
.... Back in S&B Again
Traveled 8 yr in a 40' 2004 Newmar Dutch Star Motorhome
& 8 yr in a 33' Travel Supreme 5th Wheel