Gordon,
I've updated the battery document to show the relay method.
This disadvantage of this method is that you do not have the flexibility to use the second battery to start the engine, or to use the starter battery to run the aux instruments. My advise is to keep things simple and flexible, and so use the method shown in Diagram 2, using a battery selection switch. Adding relays just means another thing to go wrong, and I prefer the option of using either battery in an emergency.
Paul, yes, your main power feed cables may be underated. This would cause the problem you describe, but it could equally be a resistive connection somewhere. Was the problem there right from the beginning or did it develop over time? If it developed later then I would suspect a bad connection.
What size cable should you use?? I would had up all the max current ratings of your various pieces of equipment, and then double it for safety. This will then give you the current carrying capacity that you need for your main feed to the aux equipment.
NOTE - the connections between the two batteries and to the battery selection switch and engine should be at least 300A. If you use smaller cables between the two batteries the wires could melt when you try to start the engine from the second battery.
cheers
BF