The simplest form of battery isolator used in motor homes and such with 2 batteries is nothing more than 2 large diodes mounted in a large heatsink. The 2 diodes have their anodes conected via metal bar, with a bolt on the metal bar connecting to the alternator. Each battery is connected to one of the cathodes of either diode. This allows the alternator to charge both batteries, but the diodes prevent the loads on the "auxillary " battery from discharging the " cranking " battery.
The only problem with this system is that some alternators rely on the battery to "excite" the field winding on the alternator, and with the diode in the circuit this won't happen. Some Delco alternators are like this. Easiest way to tell if the alternator requires system power to function is if there are more than one secondary power connection to the alternator, besides a " charge" connection for an idiot light or ammeter. Isolators are made to get around this problem, they are only slightly more complictated.
These devices are actually quite cheap to buy when you take into account the price of two 100 Amp diodes, a large heatsink, and the fun of building one. The diodes need to be rated for atleast 50% more than the full load rating of the alternator, to account for the ambient temperature under the hood of a vehicle, where the device is likely to be installed.