Take HWY 5 ...
We drive back and forth to Des Moines all the time. It's easy enough to tow. Through Knoxville, you've got two lanes each direction, between Knoxville and Albia, only 1 each direction, but you can pass if you need to. Once you're through Albia, you get two lanes, then back to one with passing lanes built in every once in a while.
Once you get to Moravia, there is a flashing yellow light over the road & a bank on your right (not much else, except some houses on the left and a shop called "Angel's" a ways past the light and on the left) ... turn right at the flashing light.
Continue straight, you will pass signs for Buck Creek Marina, you will pass the main entrance for Honey Creek Resort, then you will go through a small town called "Iconium" ...
After that, there is a sign that says State Park, with a left arrow (you're headed west, turning south .... for those coming in from Nebraska or I-35, you'll be coming east, so you won't pass the resort) ... your last minute sign is a small, square, brown sign with a rowboat figure, if I remember correctly ... turn left (or right, for you I-35 people) there, onto 160th Ave. Take that road all the way through, until it dead-ends at a stop sign - you'll be in the State Park at this point. Turn left ... you'll see the campsite check-in stand on the left - the South Campgrounds are on the right, Cabins on the left ...