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There is only one National Park in Maine; the very popular Acadia NP.
At all the campgrounds in Acadia NP you will have neighbors, too, even if they weren't already booked. There is no dispersed, back country, or out-of-bounds camping in Acadia and no overnight parking in other than designated parking areas. Acadia has a 60 day rolling window for advance reservations, and if you keep checking, you may snag a recent cancellation.
No, not all state parks in Maine have campgrounds. Most or all of those that do, though, have reservation systems. Some state parks in Maine have open field camping; others have more designated spots in the woods, and at all I know about advance reservations are recommended.
Interest in outdoor recreation and vehicle camping was on a fast track before the pandemic and has only increased dramatically during the pandemic. Some New England states, like New Hampshire, allow reservations up to 11 months in advance. You can bet all the popular sites are scooped up as soon as they become available, 11 months out.
State and National Parks all across the country are reporting the same thing; record attendance and interest. As I wrote above, best luck you have in many places is to snag a site that just cancelled, and that takes diligence, patience, and time.
Having a stock Outback is not limiting for most sites in New England. I go everywhere I want with my rwd van and trailer.
For Maine and New Hampshire for the foreseeable future, I'm going to be camping as much as possible on property belonging to friends I've made throughout recent connections.
Good luck with your search.
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