It's really this simple.
Connect your batteries like below (previously posted I'm sure).
Note the batteries are connected together (parallel) and the positive and negative wires run to different batteries. As far as the battery gods are concerned this is one big battery, not two. Ignore the other terminals.
Connect ALL the Positive wires to to the top battery and the single Negative wire (there should only be one) to the bottom battery. Connect that Negative wire to the disconnect switch and from the switch to the frame (ground). Switch the ground. One wire into the switch. One wire out of the switch and then to the trailer frame.
The top battery in the diagram is where all (meaning every one of them) the Positive wires connect. Nothing but the jumper connects to the bottom battery Positive connection; likewise nothing but the jumper connects to the top battery Negative connection.
Turning the disconnect switch Off -- wait for it -- disconnects all the batteries from the system.
-- Chuck