Not sure about that switch set up how it would be wired. I'm sure others here might have some suggestions.
However the relay that I was talking about was this kind. It is a normally open relay with a signal coming fom the ignition. You can buy them anywhere. Crappy Tire, Napa, you name it. What I did was use 10ga wire from the battery + to #30 and again from #87 to the other battery.
Here is a basic schmatic for you to follow. Just ignore the headlight and other stuff that they have listed for #87 and your battery goes in place of that.
I used this configuration for about 6 years without a problem. Sold the truck to a guy who's still driving it today with the same wiring. That's about 10 years later. So it does work and it hasn't failed yet. Even if the relay burns out sometime then you just buy another one for a buck or two and away you go.