you can overnight at LL Bean in freeport for free....dry camping, but has advantages of being near outlets....
there are also two campgrounds in freeport....
there is cabelas in south portland, ME, with a dump station....bayleys is in scarborough, which is basically south portland....
portland has some good eats and a few things to do.....
there are campgrounds in wells and kennebunk which are nice....bayley's in
also old orchard beach (like the jersey shore, i think, boardwalk, etc)......old orchard beach would be good for a couple three days....