Correct Way to connect 5 car batteries in parallel
You should put the disconnect switch in the positive lead from the battery bank.  The negative terminal is normally connected to the vehicle frame.
For marine use (and good practice anywhere) the American Boat and Yacht Council recommends a fuse within 7 inches of the battery positive terminal - a bit difficult to arrange when you have so many batteries.
I'd suggest two fuses - one for each group of batteries.