I've used campsites, and also just parked at the roadside, both in towns and out in the sticks in laybys. If you're out of the way, you're unlikely to get disturbed. As long as you find somewhere to empty the porta-potty of course. campsites are like campsites for tents- the popular ones get booked up, so for high season, school hols etc., and honeypot locations, at least phone in advance.

And watch the height bar you so commonly find at campsite entrances, especially abroad- I saw a bloke drive his car straight through, followed by a brand new caravan, neatly slicing off all the vents on the roof.