If you are coming to Wisconsin and looking for some nice day paddling or canoe style car-camping I would recommend the Willow flowage and/or the Turtle-Flambeau flowage.  The Willow is smaller, with less development on the shores.  The TFF is 14-17,000 acres (depending on who you ask) with the eastern side mostly undeveloped.  Both are managed by the local DNR and have free first come-first serve camping.  I understand you are looking for trailer access.  While I don't personally have any experience with that in the locale, I would contact the Mercer, WI chamber of commerce (715)476-2389 and check for local facilities around the TFF.  Not sure about the area around the Willow.
The TFF has been my "home-away-from-home" for years.  One year I spent about 50 nights on it between May and November.  I have heard wolves there frequently and just saw my first bear last month.  Loons, osprey, eagles and myriad other birds are common to the area.  I have seen mink, fishers, otter, beaver, etc.
All of my biggest pike were caught on the TFF, despite what the non-belivers would have you think.  The small mouth can be amazing.  Though they call it a prime walleye water, I do not have much luck with them on the eastern side.  In the central waters, live bait has produced fair walleye action for me in the past.  Many people fish the TFF for perch and it is known for it's musky action as well.
The Willow can be interesting in the summer and fall as they draw the water down.  I have been there in September when it looks more like a river with many odd bays than a lake.
Fishing on the Willow can be very good for northern and walleye--bass are hit and miss.
All this being said, I am reluctant to tell people about favorite spots so I implore you to treat these places gently if you visit.  In just my years of visiting I have seen them degraded and abused.  I believe that most of those that visit this site with any regularity are the sort of folk that I would welcome onto my favorite island on the Turtle flowage with nary a qualm.  
Good luck, regardless,
gp