Harbortown is definitely a nice campground...we have been there many times. But, it will add another 30 to 45 minutes to your drive time to Frankenmouth.
You might want to switch those two destinations around, i.e. camp in Birch Run or Frankenmouth and make the day trip to Greenfield Village. If you do that there are two really good campgrounds in that area. In Birch Run there is Pine Ridge. This is a quiet, heavily treed campground with virtually no recreational facilities. Just quiet. Joy and I are going there this weekend. At the other extreme there is a Yogi Bear campground in Frankenmouth. I'm told that is very good also. Very family oriented.
BTW, the drive time from Dearborn to Frankenmouth is about 2.5 hours.
You'll not regret your trip to Greenfield Village, either.
Rick (K8RWG), Joy, and Daffy (the spoiled Calico cat)
2014 Rockwood 8315BSS
2016 F250 SC LWB 4x4 6.2 3.73
Reese Dual Cam