We just use a hitch lock. For the generator, I either put it away, or use one of those heavy duty u-shaped bike locks and secure it to the trailer frame.
No one can stop a determined thief. But my philosophy is just to make our stuff look a little more difficult to get than an easier mark somewhere else in the area. Besides, most thieves will be a little wary of getting discovered in the middle of their escapades when an owner returns. Can't exactly make a fast getaway on a dirt road pulling someone else's rig.
In some ways, tent campers might be a little more concerned as it's far easier to just throw sleeping bags, tents, etc. in the back of a vehicle and speed away.