The only flaw with just disconnecting and reconnecting the switch is that you really need two of them, one for each battery, AND a way to prevent the main battery from getting very low.

If you kill the first battery you don't want to just "engage" the second one as it will be getting drawn down by the dead one while trying to start the truck. A dead battery can really absorb some amps.

So now you have the full battery discharging into the lower charge battery with the alternator trying to charge them both, and an alternator really isn't designed to charge a dead battery.