lezmark, the link between the house and chassis battery is in that box. It is some 3/8" busbar between the "Salesman" relay and the isolation relay. If you can get to the connections you can remove it, but it does not bend so you'll have to remove both ends to get it off.
How about this, find the wire from that box that goes to your chassis battery and try disconnecting it from there? IIRC, my Ford-based chassis has a wire from the box to a hex bolt on the battery terminal connector, it should be easy to disconnect it there. Of course, you'll have to tape up the wire well so you don't accidentally short your house batteries! You can always put your switch in there.
Stewart, Brenda and kids
2008 Newmar Canyon Star 3410