Hobby Haven is in a strip mall and they share the parking lot with other businesses. A few years ago at the Shootout I saw 1 "pop up trailer" but otherwise I don't think there is enough room for you to camp. But you can call the store and see what they say @ (515)276-8785.
Otherwise Saylorville Lake has some campgrounds that are open to the public and you can make some reservations online:
http://www2.mvr.usace.army.mil/Saylorville/Camping.cfm
I checked out the sites and it looks like the closet site is the Bob Shetler Campground which sits below the dam. Here is link to a Google map from Haven to the campground area:
http://goo.gl/maps/rwTWl
As you can see you are only 10 miles away from Haven.
Finally, there is a Cutty's campground but they aren't open to the public. But if you are a member, affiliated with one of their partners, or are staying with a member then you can camp there. Here is a link to their contact info page:
http://www.cuttys.com/contactus.html
Hope this helps!