isolators are diodes. A diode has a voltage drop across it of roughly .6 volts. A diode is like a one way valve for electricity. A voltage sense wire on the input side of a diode will never sense the battery voltage. It will only sense the output voltage of the alternator
If you have a smart regulator
that is separate from the alternator with a voltage sense wire it can be attached directly to the battery. The regulator
will then sense the voltage properly and also compensate for the voltage drop from the isolator.
An alternative that has been available for many years now is an automatic charge relay or "combiner". Anytime the combiner senses a voltage from a charging
source it will combine the two battery banks. When the charging
source stops the batteries
I prefer the combiner method and have had one installed for about 20 years. They are easier to install and will support all charging sources, rather than just the alternator.