I make that drive a lot, as Doran said. Going down 69/S.E. 14th would be ridiculous - multiple vehicles, each with a trailer ... you're asking to have the group separated by lights or crazy drivers.
Doran's map is the most direct route coming from up north ... Take 65 around the east side of town.
I don't think there's a big difference in distance or time taking highway 5 vs. 69 ... 69 is the larger road and you'll make a right turn into the State Park. Coming on Highway 5, you'll turn right in Moravia and then left into the state park (after you pass the resort entrance).