Chris replied:

Hi geoff,
Although I have a diesel 80 cruiser, I thought you might like to hear of a simple but CHEAP way to have dual batteries. Our local auto-elect simply made up a lead that connects both positives to each ( eg in series) and on the spare battery ther is a simple turn off connector, BUT the trick is, he installed a one way diode so the spare battery trickle charges even if isolated manually from the system. It's not fool proof coz you have to turn it off yourself when you intend staying in one spot for a day or so, but costs only 20 bucks or so and nothing to go wrong. It has worked well for us for over three years,including a 6 month trip last year. Cheers, Chris