If I am on the same page you are, what you have is technically not referred to as an isolator. The catalogs call it a "battery selector switch". Since it serves the same function as an isolator, what you are doing is manually choosing which battery you are using or charging, although there are dozens of ways to wire it in.

I am not exactly sure what function it had in your truck. Isolators are generally passive, so you do not have to do anything to manage electrical flow. You can wire you voltage gauge with a selector switch so you can choose to read either battery, or your can simply tap the terminal feeding the switch in which case you will be reading alternator output into the switch.

I am not sure what you want to achieve with your truck, so it is difficult to be precise in my answer.

Steve