Hi MarkCHI,
Sorry to see not much of a response to your question. I myself am not familiar with the southern section of the Trans Wisconsin Adventure trail, but have done the top half from the Black River State Forest north to Lake Superior a couple times.
On the northern half there are lots of places to camp, and in the Chequamegon–Nicolet National Forest there are lots of places you can boondock, as well as some neat primitive campgrounds. They don't see a lot of activity until Memorial Day Weekend, after that they can fill up on the weekends. My favorite time to hit that section of the trail is before Memorial Day and after Labor Day.
I typically spend a night at the EAST FORK campground in Black River State Forest. It is a primitive campground and quiet.
From there I take a slow drive along the trail until I get in the National Forest and then camp at the Stockfarm Bridge Campground, or if ahead of my schedule I will hit the Beaver Lake campground which is also primitive. I like to be at camp and setup so I can enjoy an evening around the campfire relaxing.
After that I head north along the trail and usually camp on the shore of Lake Superior at Detour Point. There is a casino there with campsites that kind overlook the big water.
Hope you have a good trip. I am heading up to the MN Arrowhead next month to hit some of the back trails to camp, canoe, fish for lake trout, and chillax like a boss.