.
If one does install their own 12V battery switch, please "properly" install it. If installed incorrectly and one forgets to turn it ON before towing, one's RV will NOT have emergency break-away trailer brakes. This is both dangerous and illegal.
To properly install a manual 12V switch, install on the 12V "+" LINE (which only cuts power to the chassis's 12V distribution system) and connect its 12V break-away break wire to the battery "+" post (or to always live side of the battery switch). For picture, see below diagram...

Hope this helps as well..