Great thread
Well technically you can't go off-road in the state/national forests in MN any more and technically you can't block a roadway. So I guess the only way you can legally camp is at the end of a spur. Since most of the spurs have been closed to vehicles it is now incredibly difficult to find a fully compliant place to spend the night that isn't a full-on campground.
Pretty much all of the parking areas in WMAs and near county lands have been posted no camping.
The two things I would suggest:
1) As noted upthread the non-electric parts of state park campgrounds are ideal for truck camping. These are usually smaller, quieter, more widely spaced sites with no electric. Also true of the newer county campgrounds around the metro e.g. Lebanon Hills (Apple Valley) and Bunker Hills (Coon Rapids). The overflow campgrounds that are nonelectric only are particularly good, like Carley State Park.
2) There are some smaller county/municipal campgrounds with no services that are typically remote and quiet. No water or electric or dump station. Little American Falls is one of my favorites. These don't appear in directories so you have to check the county maps/websites or ask around.