It has been a few days since my last update, due to other university modules that required my attention. However, I am trying to balance it all and make sure to keep developing my mechanic for the January deadline.
Today I had a major breakthrough and fixed a large number of issues with my project. Previously, I was using a Unity hinge joint in order to connect the tractor to the trailer, which is what led to the unexpected and buggy behaviour. After thinking through the problem more logically I decided I was trying to make it too difficult.
So instead, I scrapped the old version and recreated it, but using physical 3D game objects as a colliding tow bar that is hooked in. This has completely removed all the buggy behaviour and allows the vehicle to behave in a more realistic and easy to controll manner. I recorded a short clip which I attached to this tweet.