You'll do/be fine coming down 15 to 17 then 9. There's a nice market at the 17/9 junction, otherwise stopping in Cedar City or staying on 15 to 9 and driving through Hurricane are your best options.
Leaving, you'll need to head West on 9 again, then catch 59 as already suggested as your route to Kanab. Hopefully you'll be able to go visit the North Rim as part of your stay in that area.
The Zion River Resort is a very nicely managed park. I'm sure you'll enjoy it. Depending on when you're there, you'll be very close to the road to Kolob Reservoir which passes through parts of Zion. It's off the beaten path in a good way, but they have a major construction project on that road this year. You'll also be close to the ghost town of Grafton, where they filmed the bicycle scene from Butch Cassidy and the Sundance Kid.
If you'll be visiting Zion while the shuttles are running, it can be just as easy to park your car in the upper end of Springdale and walk through the park entrance rather than trying to park inside.
2013 Heartland North Trail 22 FBS Caliber Edition
2013 Ford Expedition EL with Tow Package
