The State of Minnesota has a pretty decent system of state parks with camping. Here is the site to check for availability and to reserve in advance.
http://www.stayatmnparks.com/SearchPark.aspx
I quick checked Afton Park which is pretty close to the convention and since it is Labor day weekend it appears that it may be filled up to the 30th of August. But after that it appears they have space.
You can reserve in advance and most of these parks have bathrooms and if you get a "Group" space or spaces the price can be as low as $3.00 per person per night. Check the details but soon as Mcdufus may just go and reserve everything to keep us out. i can't make it but Hope it is a success.
But be forewarned they say "No Alcoholic beverages" allowed. And the fireworks will get you kicked out. Nothing about political signs though so be sure to bring those.
You could probably google minnesota or even St Croix river camping and find some private campgrounds that could give you a good deal starting the 1st or 2nd.
The St. Croix area is really nice and only about 45 minutes to St. PAUL.