Just to throw in a little support for Allistair, I have the same, or very
similar setup where all of my electronics run off of the house battery
except for the starter.  It has been a year or two since I wired it up, and
I did have some circuits that were bridging the two batteries but I was able
to eliminate all connections of the two batteries except for the relay that
connects them when the van is running.  I did test this.  Works great,
especially for someone like me that leaves the lights etc. on sometimes,,
even parking lights can run the cranking battery down after a while!

Jonathan Poole
'82 ac westy