Or, instead of a switch, a buck converter between bank and 12v panel. This way, if charging is automatic, "higher" voltages (above say 13v) will never reach the panel. It's such a simple fix, it seems.
I have looked at many rv wiring schematics, and occasionally I see one with converter power going thru the panel first, before going to the bank. Perhaps this is how Phil's rig is wired? If so, he would have to make considerable changes to his wiring layout. But, if most rv's are wired such that the converter power goes to the bank first, and then to the panel, then a buck converter would be a very simple fix.
converter to bank to panelconverter to panel to bank