I have installed versions of this type of switch on my off road vehicles and travel trailers for years. Simply install the switch between the main ground and the main connection to the negative battery post.
The switch is designed to handle the arcing that may occur when turned, it is weather resistant, and can be mounted to the battery box itself, on a bracket on the frame, or virtually anywhere else you want to place it. I'll snap a picture of the recent mounting I did on my smaller travel trailer and update this post with a second image in the next day or so.