Sorry but MOST of this is not true. A "battery isolator" is nothing more than a pair of diodes. They do not magically figure out which batteries "only need charged." IN FACT back in my parts days there were a couple of VERY DANGEROUS failures because of isolators. The diode supplying the main battery runs hotter because that battery sees more draw and use. So that diode tends to fail, and it sets up the following condition:
If the main diode fails, the main battery starts to drop and that is where the regulator sensing is connected. So the regulator ramps up charging voltage to a higher and higher level, trying to bring up the main battery. Since there is no longer a closed loop so to speak, that can get fairly high. Meanwhile, the poor secondary battery is over there bubbling furiously with overcharge
In my opinion you are WAY safer and better off with a constant duty solenoid. It ties the batteries together and tries to equalize their voltage, and can also be used as a "jumper" if you leave the lights on eg and run the main down. It can fail, but will leave the alternator properly connected to the main
There is no doubt that a bad battery such as one with a low/ dead cell can screw things up