Not sure it helps, but attached my wiring diagram. I'm just installing a Perko switch, it shows how the Perko switch goes into the system.
As the last post says, batteries go to posts 1 and 2. Then install a heavy wire (minimum 6 gauge) from Common to wherever your two batteries used to connect (in my case, a positive bus bar).
My reason for installation is to ensure the last remaining phantom draws are eliminated. When I turn the Airstream kill switch off, there are still a few small phantom battery drains (primarily the LP Gas detector), that are big enough to put a serious dent in the battery over a month.
2007 Classic 25'/ 2013 Cayenne GTS/ 2015 GMC 2500HD Denali