On 28 April, Maine's governor Mills released a staged reopening plan, which includes campgrounds. In short, campgrounds may open on 1 June for Maine residents and those from out of state who have quarantined in place for 14 days. Campgrounds, on 1 July, will be open to visitors and Maine residents.
The full reopening can be found here: https://www.maine.gov/covid19/sites/...onomy_Book.pdf
Some of this is being challenged by industry groups, so subject to change. As well, any increase in infection rate would effect these dates as well.