A locked truck or camper door will only deter the ''honest'' thief. Break a window and you are easily into either.
The best thing is to keep things out of sight, not only when parked but also when you are around the rig. A proper thief will be scouting vehicles, and this is also done when you are at the vehicle. I never pull anything out that is not going on the hike with me, I will often stop ahead of time if I need to sort through gear, then move on to where I will be parking. But with a lifted truck and a camper I am easy to spot and follow, and spending two weeks at a time in one location makes keeping the gear a secret difficult. Be as carful as possible and hope for the best.
That being said, if the person is in a meth induced desperation, they will break in no mater what.