We always lock up the MH when we are going to be out of sight of it. And like other have said, electronics and anything we really don't want walking go inside. We usually leave the chairs and tables out. We've left a lot of stuff out at times and at other times very little. At the racetrack, we leave out more things; so much valuable stuff is left out at every pit anyway.
In a lifetime of camping, I don't recall much gone missing. When you're reenacting, the only way to secure a tent is to tie it closed and that's usually done more to hide non-period items and to keep any wind from making the tent into a sail. Usually someone stays in camp to keep an eye on things.