The only reason to install a battery cut off switch is to shut the power off when in storage so the battery doesn't go dead from parasitic draws such as LP detectors, radio memory etc. The convertor in the trailer will automatically shut down before the battery is overcharged. If there was going to be a problem with overcharging the manufacturer would have already installed a switch. That being said, if you still want to install one it won't really matter which side you connect it to as once the battery connection is opened the battery is out of the circuit. It has to have both terminals connected to function. I installed mine on the positive side but it really doesn't matter.
B.O.