This was covered very well in a recent thread.  The subject line was:
"Switch circuit to Aux Battery ??"  You can find it in the archives.
Edward

At 08:33 PM 7/16/2004, you wrote:
>    I'm sure this has been done many times. I put an couple of auxilary
>batterys (deep cycle) in my westy, fed by a relay which isolates them
>from the starting battery, and now I want to switch the light over the
>sink, pump, stereo, etc. over to the auxilary batterys.
>    Is there a simple way to do this? Any websites with good diagrams
>or pictures, like there are for hooking up the relay and aux batt
>itself?
>
>  --
>Harmon Seaver
>CyberShamanix
>http://www.cybershamanix.com
>Hoka hey!