It has nothing to do with aluminum being common/ground.
The steel frame of the trailer is also part of the common/ground plane. The negative wire in the UCord is connected to the steel frame of the trailer in most cases. As is the negative side of the battery in the TV.
Just as the steel or aluminum components of the TV are part of the electrical circuit on a 12 volt
In reality the negative side of the 12 volt
DC circuit is not GROUNDED. It is simply the COMMON side of the 12 volt DC circuit. There by the return path for most DC devices is the metal in the trailer. It saves wire and it saves weight by literally cutting the amount of wire used in half.
The only true ground in the trailer is provided by one of two sources. You either drive a ground rod (which by the National Electric Code must be a rod 8' long made of a specific material) into the ground at your site. Or you connect the shore power cord ( properly wired) to a properly wired power source. In the latter case the utility company in effect has driven the ground rod for you.
The use of the term GROUND should have never been used for the COMMON side of a DC circuit. IMHO