You're confusing people and contradicting yourself - in the headline you claim the data would be applied to the camer, in the text you talk about a Null. anyway, assuming you hgaven't done anything obscure we can't know, the bahvior is correct in that the tracker will only create a camera and doesn't care about any parenting tricks. Other than that when you let the tracker create a 3D layer on a tracking point, this has no influence on the camera, but naturalyl there may be random flipping or odd rotations if your scene is generally problematic like having odd camera motion or gnerally the camera being very close to some of the points, so gimbal lock issues appear. Impossible to tell the exact reason without seeing your setup.
The 3D camera tracker solves the scene for camera data and trys to put a virtual camera in the scene that matches the movement of the camera that shot the footage.
If you want the truck to obscure the text as it passes through the scene then you have two things that must be done. The first is to remove the tracking points that the motion of the truck introduces into the scene. If there is a bunch of motion from the truck it will throw off the camera tracker because the 3D camera tracker is looking for things that don't move through the scene.
The second thing you must do is create a matte for the truck so so you can create another layer that just contains the truck. You put your text between the masked truck layer and the background. You can create that mask either by motion tracking the truck in AE or Mocha, or by using the RotoBrush.
If the object is something that does not move through the scene like a tellephone pole and you want to put your text behind the pole you can simply add a 3D solid to the scene where the pole is in one frame and use that for a track matte for the text.
I hope this helps. Maybe this little instant tracking demo I produced a while ago will help:
Thank you so much this is perfect!