You have a lot of options. From Williams you can get to 89 through Flagstaff or you can drive into Grand Canyon Nat'l Park on 64 and continue on 64 east to 89 bypassing Flagstaff. It's a good easy drive but try to leave early morning through the park before traffic.
Then north on 89 and near the junction of 89A you come to a detour sign. It's a good detour running roughly parallel to 89 and ends at Page. About a year ago there was a landslide on 89 and the road is still closed. From Page you will pick up 89 again and take it into Kanab, Utah. This is all good driving for RVs with hills but not mountains.
Or..take 89 north to the Jct. of 89A and then take 89A to Jacob Lake & Grand Canyon North Rim if you want to stop there. Then continue on 89A to Fredonia, AZ or Kanab, UT. On 89A before Jacob Lake you'll have a steep curvy climb so it's more difficult than 89.
Since you have a 38' RV you could then take 89 from Kanab to Hwy 9 and west into Zion. However, there are restrictions so you'd probably have to disconnect the towed vehicle and drive separately through the tunnels. Here's a link for the tunnels:
http://www.nps.gov/zion/planyourvisi...mel-tunnel.htm
They call it escorted through the tunnels but what happens is that they stop traffic at both ends and allow you to drive down the middle of the tunnels where they're higher. Many RVs do it. After the tunnels you'll have a steep descent down to Zion. Just drive it slow like everyone else.
An alternative route is from either Kanab, UT or Fredonia, AZ, at Fredonia pick up AZ 389 which becomes 59 at Colorado City and continue into Hurricane, UT and then east on Hwy 9. No tunnels this route and it's an easy drive will some small hills.
Have a good trip!
Flaggstaff > Zion
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