At last I have sorted the fridge wiring issues.
I had one of those what if moments thinking is the large relay between the batteries is connected to the charging and is the earth switched as per the rest of the charging system. With the engine running the relay is switched via its earth. The other wire from this relay was split in two one not being connected to anything.
The 12volt motorhome wiring is behind the rear battery, Representing a rats nest of odd coloured wires one had a battery live and a fuse. The wiring to this fuse is red, then its green and yellow after the fuse. Connecting this wire to the green and yellow in the relay wiring and starting the engine the fridge now works on 12volt. At some time on the last trip away this wire must have disconnected from this fuse.
. I can only think that this relay disconnects the main battery after starting.
The motorhome wiring will get a good dig out next. I may remove it and start fresh with a proper junction box instead of stuffing loose wires down the back of batteries.