I have found that you can do 1 of 2 things, or both to position the camera out a bit more.
In the trucks XML, you can add the code "RadiusMultiplier="1"" to the <camera> line to look like this <Camera RadiusMultiplier="0.8" />. The higher the radius, the further out the camera is. The distance the camera zooms in and out remains the same, be it just further away from the truck.
There is a class folder called 'camera' in the Media.zip. Find the XML for camera you use (truck centred or original) and then edit the camera location.
The camera location code looks somethine like this:
Eye="(0.5; 2.0; 0)"
Dir="(0.93; -0.2; -0.2)" />.
The "Dir" line dictates the camera direction, so if the first value has a minus before the number it will be pointing backwards and any positive values point forward. The second value is the height i.e. which direction the angle of the camera is pointed up or down , and the last value is the direction left or right. In the example above the camera position would be pointing forward, but also slightly down to the right in it's default position. This should help identify which camera code to edit.
In regards to moving the camera to see more of the trailer, you can move the centre location of the camera by editing or adding a parent frame in the camera section of the trucks XML.
So it would look like this:
<Camera RadiusMultiplier="0.8" ParentFrame="chassis_rear"/>
If you're not sure what parent frame to choose, you can look in the trucks xml, you should be able to see what the what frames are present in the model, and you can just try them all to see which frame will put the camera in the best position. There should be frame called "bone_rear" or "chassis_rear" or something along those lines which could move the camera centre posiiton to the rear of the truck.
This is all stuff that I have worked out by messing about with the XML's so probably aren't the official or best way to do things, but it works for me.
Bare in mind that the LOD draw distance in this game is extremely short, so if you position the camera too far out, some of the sounds and visual effects (including the mud clumps) won't appear as they should due to being outside of the draw distance. I assume that this is the reason why the camera is so close, and I guess it also makes everything seem bigger too which adds to the atmosphere. I tend to just edit the trucks' radiusmultiplier as it quicker and relevant to the truck without effecting the others in the game.
As with all modifications to the game files, make sure you back them up and then once you're happy, back up the modded files so that when another update happens, you can easily put your edits back in the game.
I hope this helps.