Jason I don't think any of the public ramps allow overnight parking but you could probably just park there and not be bothered. Or call Howards Point Marina who has a ramp and see if they would let you park overnight. Last Time I used their ramp(a few years ago) it was $30 to launch.
There is a nice cove on the SW side Wawatosa Island which is perfect for overnighting. That's pretty much where i go anytime I'm overnighting with the boys in the SeaRay. Wauwatosa is located on the southwest area of the lake(Smithton Bay) and is public land so you can go ashore if you want. It's off the beaten path & well protected. Lot's of channel markers & buoys marking rock piles to get in/out of it so recommended to approach and anchor when it is still light out until you get to know it.