89T is the detour temporary road and as stated, it will be just fine for your RV.
If you get into the Flagstaff area during the morning and want to find a campsite for the night, there's a lovely one just north of Flagstaff and just off Hwy 89 where the sign directs you to Sunset Crater Volcano Nat'l Monument. Turn right and drive about 3 paved miles and Bonito national forest campground is on the left a few hundred feet before the entrance booth of the Sunset national monument. It's dry camping, paved, flush toilets, no showers, no dump but it is in a beautiful setting with large sites. There are no reservations and it does fill by 3-5 pm but most leave the next day. It's usually a night or two-type of campground. Get there early and you'll get a spot.
When you leave the campground you can then turn left and go through the national monument's entrance booth and drive along the road viewing black lava and viewpoints. This is short loop road which will bring you back out onto Hwy 89 for the continuation north of your trip to Page. Also along this loop is a Painted Desert Vistapoint and Wupatki National Monument for nice Indian ruins tour.
