Yes, 89 leaving Zion northbound is fine for RVs.
Grand Canyon east entrance is currently still closed. I assume you're looking at the Nat'l Park websites to keep up to date. Leaving GC, you can take 64 south to 180, but that dumps you right into the town streets of Flagstaff. If you don't mind driving through town, you can go that way. Alternatively, 64 south to I-40 is a simple drive.
Along 89 northeast of Flagstaff, if you have time, you could stop at Sunset Crater/Wupatki Nat'l Monuments. You can look it up online.
Regarding 89A or 89 closer to Page....89A takes you up a mountain about 3,000 ft to Jacob Lake (pine tree elevation), then down the other side. The other way...I've stayed a few times overnight at Page Walmart. 89 from there is an easier flatter drive. Both are scenic. Several miles north of Page, the Big Water BLM Visitor Center used to have a cool dinosaur exhibit. I haven't been there in a long time, so don't know if they still have the same display.
I don't know if the places I mentioned are opened or closed.
Have fun!
