Most areas of Zion do not allow private auto traffic, so you will take the lovely shuttle. Grand Canyon South Rim has plenty of RV/bus parking, and you will walk to the shuttle and tour most of the South West rim within the park. So no problem with your route in the RV. If you go from Zion to Grand Canyon try to route through Monument Valley and spend a night at Goulding's and take the tour through Monument Valley. Spectacular. At the Grand Canyon we often basecamp at Circle of Pines KOA. Large lovely campground, heated indoor pool, large sites, about a one hour drive to the South Rim on a very good two lane road. Also, spend a day at Bearazona, very close, and use their car(s) to drive the wildlife park. Williams is a nice town with some good restaurants. The KOA has a contact for a local car rental company which we used on one of our trips. Williams is central for day trips to Sedona, Flagstaff, and Sunset Craters. All fun. Happy Trails.