In west lower Mich there is the Platte River that goes to Lake Mich, Sleeping Bear Dunes, Suttons bay, Tall Ship Sailing Co on West Traverse bay. The ride north along lake Mich and through Harbor Springs is nice. The Traverse City State Park should be slowing down after labor day.
I would go to the island via Saint Ignace. Much nicer ride imo. Check the schedule as I believe one of the morning boats goes out by the bridge before hitting the island. You shouldn't have a problem with reservations at that time of year.
MichTaquamenon(?) falls are nice. To the west of there is Grand Maris with a huge picturesque bay. Sable Falls are near GM.
Farther west is Munising and the pictured rocks. They have a boat ride out along the rocks that leaves from downtown and another smaller glass bottom boat that does the old ship wrecks. There were hundreds of wood ore boats lost in the Superior storms. I believe there is a hiking trail to the north east of Munising called Mosquito falls that goes out to a rock ledge shoreline. Minors Rock and beach are also up that way.
We are stopping at the Casino in Christmas on Sept 8 or 9 and then checking out Bond Falls flowage to the west and south of Bruce Crossing. I believe the Sturgeon River Gorge is over near Baraga.
I have been to the Keweenaw peninsula and Copper Harbor but I would investigate it before pulling my FW up there. They actually considered making Calumet the capitol of Michigan during the copper boom. I dont want to tell you to go up to Copper Harbor without knowing about rig size. I'm sure it's doable but? Porcupine Mt and Lake of the Clouds are also nice.
I went to Seney long ago but dont know how accessible it is. They have some rafts or boats that go out over spring feed ponds that are crystal clear.
The drive along Lake Michigan is also nice. Lots of sandy beach areas west of Saint Ignace and also a pull out at the cut river bridge roadside park. It has a walk way and steps down to the lake.
I read your post again and with 5 to 6 weeks you may want to look into the Keweenaw a little more. Rich history in copper mining. I know the last and farthest park is Wilkins at Lake Fanny Hooe. They are driving semis up there so it cant be a problem for RV's. I just remember some twisting areas that seem tight. probably just my memory as it was at least 15 years ago.
Just my opinion but if you want more of a wilderness type trip do the UP. You may even do the Soo Locks boat tour if you are into that type of stuff. Check the closing dates of some of the areas as the season goes on.