Honestly, I don't like to leave my Jeep and trailer unsupervised for any length of time. Too much stuff attached (including the trailer!) that could disappear in short order. If stopping to eat at a restaurant, I park it to where I can see it from inside the restaurant. If I'm camping, I'm there with it (unless I go to a latrine, and then I'm gone just a few minutes). If running into Walmart or a grocery store, I park where there are no other vehicles around, and make my trip quick. I'd never leave it at a trail head unless I was with a group, and someone was staying behind. And I'm paranoid if staying at a motel with the Jeep and trailer in the parking lot.
Honestly though, the place I'm least worried about it is at an organized campground where there's a fee to camp. I figure most folks are there to get away, and nobody's going to mess with you.
And I sleep easier knowing I'm armed in some way (firearm, bear spray, etc.).
For the record, I've never had anything stolen, messed with, had any scary situations, etc.