We stayed at Rustic Barn in Kieler WI and loved it, the scenery is hard to beat. The pull through full hookup sites are VERY TIGHT and I would not want one. When we were there they offered a pump out service for the much larger water/electric sites.
The park is impressive for any American Indian history buff. You can spend as much time on the trails as you wish. If you are limited on time the visitor center is a must for an overview and you can walk out to a couple mounds and come back. Just know that the shape of the mounds are large in diameter so if you are expecting to see the shape of the bears, it will not be obvious.
It is to bad you don't have more time. The mounds at the south unit about a half mile down the road from the visitor center are impressive. There are a series of marching bears then a couple eagles. It is a couple miles back to the mounds after hiking up a step hill from the parking lot but it puts the mounds at the visitor center to shame.
We are hikers and history buffs so we spent a full day!
Since we came from the campground I can't remember if the parking lot would handle the rig.