whats the purpose of removing all of the electrics from the main battery except the starter? If you wish to run, lights, radio, heater and everything off the second battery then I would just change the original battery round with your second one unless there the same and then run a separate relay to the starter from the other battery. On mine I have everything I would leave on whilst the engine is off on the deep cycle battery (Radio, inverter, interior lights, power sockets etc) and the rest is on the main bat. On a 2 month trip round WA the main bat was perfect everytime. In a nutshell wiring everything to the new battery will be a pain. A simple tin can solenoid will isolate the 2nd bat when turned off or for $150 you can get a projecta dual bat isolator system. Like you say, earth both bats together and make sure the cable between the both + terminals is substantial. (IMG:style_emoticons/default/smile.gif
Hope thats of use..