Thanks for the replies. It seems like it has to be something with the relay (I guess it would be considered an isolator) it definitely isn't anything of the "smart" variety, you just apply +12v to the small terminal and a relay clunks in to connect the two large battery terminals.
It's in a rather inaccessible spot so I wanted to see if there was anything else that might be causing it. There isn't any way that the house draws can pull voltage backward through the shared ground of the chassis battery or something odd like that right?
