The CG you probably are trying to remember is Mackinaw Mill Creek Camping. A very large CG (200 acres) on Lake Huron with 1 mile of beach and is only a couple miles south of town/Big Mac Bridge on US-23. We/ve stayed there many times. I think they have 450 sites and for all types of RV/or tents. Beach front or in the woods or an open grassy site, take your pick. Phone No. for reservations is: 231-436-5584. They are packed of course for the bridge walk as there are 50,000 to 80,000 that walk the bridge on Labor Day. Reservations are needed very early for Labor Day weekend anywhere with in 50 Miles of the Bridge. They have gobs of busses running constantly carrying the walkers back across the bridge so that's not a problem.
The Big Mac is only 275 miles from our home and my wife and I have walked it twice in the past on Labor day. We've driven across the bridge a couple hundred times and still never get tired of it because of the special thrill! Also driven/piloted our various powerboats and even our big sailboats under the very high bridge a dozen times or so and the vehicles and 18 wheelers going over you look like little match box toys. It's only 250 miles to the bridge with our cabined boats on Lake Michigan and such fun as there are constant fine maintained harbors, docks, and towns all the way and only 10-20 miles apart! Also several islands with docks etc. The bridge walk is a good five miles of walking 1/2 uphill on the very high bridge and the other half is downhill and gorgeous sites all the way.
BTW, jumping off the bridge is not allowed, ha ha! It would be a very quick one way everlasting trip!