I don't support this type of connection due to the fact that you could blow up a battery if the second battery was almost dead and you activated the relay. It would cause a massive inrush of current from the charged battery and could cause serious damage if a plate shorted out.
The use of a dual battery isolator is the safest way to go. Any time the engine is running, it will keep both batteries charged. When you shut off the engine, only the second battery will be used to run your aux equipment connected to it.
I also ran one of these in a service van I had for many years. Never had to worry about killing the main battery to start the engine with.
As has been already pointed out, you may want to consider upgrading your alternator to be able to handle the extra load in trying to charge the second battery if you suck it down very much. A normal vehicle alternator is sized to handle the normal vehicle load, hat the vehicle has. If you add much above the normal load it was designed for, you could run into problems down the road.
An example of this is like in my Ford F250 diesel I have. I took out the alternator twice. Then I up sized the alternator. So far I have not had a problem since. The last time I changed it, it only took me 15 minutes. I have had the truck since 2004 and put a number of miles on it.
When you do go to add the diode isolator, you will need to move the connection from the output of the alternator and tie it to one side of the diode isolator. Then run a new wire from the alternator to the common of the diode isolator. The second battery goes to the other side of the diode isolator. Don't forget that these wires need to be sized to handle the charging current going to the batteries. The wire going to the common point of the diode isolator needs to be sized to handle the full rating of the alternator output.
Hope this helps. It may sound like a bunch of effort, but if you use your vehicle out in the boonies at all with the engine shut off, you will come to a fast conclusion that the diode isolator will pay for itself the first time you find you have killed the normal vehicle battery playing with your radios with the engine shut off.
One other thing that I did was to add a starter solenoid with a push button under the dash. This was used to connect the 2 batteries together in an emergency if the vehicle battery won't let you start the vehicle. I have got caught a couple of times from leaving a door open and drained the vehicle battery way down. it wouldn't start the vehicle. Pressed the emergency button and was able to start the vehicle. just make sure your power source for the button comes from your second battery.
