The Dahon Jack doesn't fold up as small as those mini-bikes, but it definitely is comfortable on lots of terrain. It has thick, bouncy tires for that. It is a full-size bike, that folds.
It is
not little, though. If your camper is tight, you probably want to go with a mini-bike. Folded up, the Jack is the size of, say, three [collapsed] aluminum lawn chairs, stacked.
I did a review of my Jack on the Folder Society, if you're interested.
http://www.foldsoc.co.uk/Mike/jack.html
Also, you might not want multiple gears, but imagine the hills in parks that you may camp in. You wouldn't want to push a bike up hills, for the sake of a pound or two. Gears are nice to have. Just a thought. I considered getting a single speed, and now I am glad I didn't. (Please, no flames from fixies!)