When I wire the 12V circuit for a trailer connection (the same thing as a dual battery set up with the battery located in the trailer) I use a regular old solenoid. These mechanical relays are incredibly robust, essentially the same design as an old Ford starter relay. They also have a very low "ON" resistance. If one does fail you can get a replacement at any RV parts store in the world. At less than $30 they are, IMHO, the best bang for the buck.
The down side is you have to use jumper cables to self rescue, but you probably carry a set of those to help out others anyway right? My opinion is you should maintain your truck electrical system well enough that needing a jump is the exception, not something you would spend time and money on hard wiring switches into your electrical system to accomplish.
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Rust never sleeps.
1975 FJ40 "Mr. Bill" not quite stock, I'm thinking about getting a hula girl for the dash.
1998 UZJ100 [strikeout]Bone stock, ready for change.[/strikeout] getting less stock all the time
1984 FJ60 What the heck was I thinking? Another Cruiser?