The other campground in the UP with the fantastic views of the bridge is Lakeshore RV Park, address: W1234 Pete LaBarbe Rd, St Ignace. Phone 906-643-9533, email address: firstname.lastname@example.org, website: www.lakeshoreparkcampground.com
Iím not sure how large the pavilion is for gatherings. If you make a trip up there it would be a place to check out that people would remember the view of Lake Michigan and the Straits.
Good Luck on your trip north to find a nice spot for the rally.