I put one under the rear seat in a plastic box.  Ran the (+) cable right to
the starter solenoid terminal where all the big (+) leads meet.  (From the
alternator and then front to the original battery.)  Ran the (-) ground
cable to the starter mounting bolt.  Really helps with cranking speed.  It
is also a Group 41 battery--I have two matching Interstate batteries.  On
the (-) cable at the battery end, I used a lever-type connector.  When
camping for any length of time, I disconnect this cable which saves it for
starting purposes when I am ready to leave.  All the original accessories
run off of the stock battery.  No wiring is mangled and no relay is needed.
Only drawback is lost space under the seat.  BTW, It  will not run the
fridge on 12V, but I use propane or 110V AC for that.

                                                                  Jay