I know there are better options out there, but with the isolator I run...my gauges in the dash show main bat @ 14v and my second bat @ ~14.4 while driving. My second battery runs my headlights, highbeams, electric rad and trans cooler fans, and anything else down the road.
Most isolators use Schottky diodes, which will be more like .3 or .4V. It's actually been this way for a long time. The voltage drop is a little higher at full load, but you still get the "lesser drop" for topping off the charge on the battery. It's simple to set up with most alternators so that they regulate the voltage after the isolator anyway. So you lose power, but not voltage - 100Ax0.5V=50W. This is why the diodes are in a heatsink. Of course that's under high load - usually the loss is much less. You lose power in a solenoid coil as well, but that is constant, regardless of load.
