If you're out in the NE of the state we stayed at Kennisee Lake in Jefferson. No problem with a 35' rig. It was a nice campground with a couple shaded areas (we were lucky to get one) and an extremely large pull-through area with little trees. There are some seasonal but they are kept pretty nice (and to a minimum). You'll be about an hour east of Cleveland and maybe 40 minutes to Erie, PA. The lake (Erie) will be about 15 minutes. I grew up in the area and stayed here for 14 days last summer.
There is also a KOA in Madison/Thompson area (about 25 minutes east). It's a pretty nice park. Madison is my hometown and between Madison, Thompson, Geneva and Harpersfield there must be 25 wineries now. (all within 20 or so miles of each other)
Geneva State Park has some RV camping but I've never been there to know how large you can go. This is basically across the street from and with access to the lake.