I have found over the years that some battery switches described above have eventually faulted by there constant use. I have changed to using the remote battery cut-out similar to part number MS-21 on Ron Francis Wiring site. These rely on a mechanical latch in the off and on position not electrical so no power is drawn.
These units are controlled by a push button that can be hidden anywhere that only the owner will only know about. No extra battery cable , just break into the existing positive cable and mount unit.
The only drawback with these units is if you have a radio/CD player with a memory. This will be lost with each aplication of the push button to isolate the truck wiring from the battery . To overcome this, the supply to the push button , which is always live as it is feed by the battery side of the MS21, can also be used to supply 12v to the radio keeping the memory active even after isolating the truck wiring by aplication of the push button. Remember to fuse the cable to the push button / radio memory as it is fed by the battery.
These units are used for example by people leaving a houseboat empty for a few days/weeks. Look around before purchasing over here in Oz they are about $80 > $50 in your money
