From I-40 in Arizona take 191 north (you could stop at Canyon de Chelly Nat'l Monument. There's a campground there and you can take a variety of tours or just drive the motorhome around the circle of the canyon.
From 191 go west on 160 to Kayenta, AZ then north on 163 to Monument Valley. Stay at Gouldings RV and you can take a tour from there.
Zion's shuttles run through the end of November. From Monument to Zion backtrack and take 160 west to 98 into Page, AZ (Glen Canyon Nat'l Rec Area w/a RV park). I'll give the easy drive into Zion from here. You didn't state your RV size.
The easier route: From Page take 89 west to Kanab, UT then south a little back into Arizona at Fredonia. From Fredonia take AZ 389 west which becomes UT 59 to Hurricane, UT. Go east on Hwy 9 into Zion. To be able to park and use the shuttle you'll have to stay either in Springdale, UT at the park entrance & the shuttle goes into Springdale, or preferably get a site inside the park at Watchman campground & the shuttle stops there.
(More mountainous route would be from Monument Valley west on 160 to AZ 89 near Tuba City then north on 89 to 89A. Go west on 89A toward Jacob Lake (this will be your steep uphill) You could also drive into Grand Canyon North Rim from here - easy. Then either 89 north to Fredonia & the above directions to Zion or 89 north to Mt. Carmel and pick up Hwy 9 west into Zion. You'll have some tunnels to go through with restrictions & it depends on the size of your RV. Read the park's website on the tunnels. After the tunnels it's a steep downhill into Zion.
To get back to I-15 take Hwy 9 west.
From Monument Valley to Flagstaff take 160 west to 89 south to I-40.
OR.... if you wanted to drive through Grand Canyon South Rim, take 89 south and pick up Hwy 64 west at Cameron, AZ (great Navajo tacos & excellent restaurant with a beautiful gift shop at Cameron). Hwy 64 west is a good drive with a RV right through Grand Canyon NP. Many viewpoints to stop at. If you didn't want to stop overnight at Grand Canyon then continue on and take 64 south out of the park to take you to I-40 at Williams.