First, 89A from Flagstaff to Sedona will be undergoing construction all summer and will close completely at night and one-lane at other times so it will be very slow-going. Also, it's not recommended for big rigs.
https://www.azdot.gov/mobile/media/news/2017/03/08/project-making-safety-improvements-to-state-route-89a-south-of-flagstaffOP, you said you were coming from Winslow, AZ and want a reasonable place to stay for visiting Sedona. I would highly recommend you take the route:
Winslow, Hwy 87 to 260 west. At this point you will have a fairly steep downhill to I-17 at Camp Verde. It's a popular RV route so it's very doable. Just downshift before you begin the downhill so you won't use the brakes as much. It's a beautiful drive. (Even if you continue to Flagstaff and take I-17 south you will have a long downhill.) From Camp Verde continue west on 260 to Cottonwood and stay at Dead Horse Ranch State Park. It's very nice and has big sites. It's an easy drive into Sedona which is a madhouse.
We've done this drive many times and stayed at Dead Horse Ranch. We love it!
