I put a second Group 41 battery under the back seat in a plastic battery
box.  Run heavy gauge cables directly to the starter, both (+) and (-).
Starts like a champ.  When camping I just disconnect this battery for the
trip home.  I use a lever type disconnect on the (-) cable.  Original
battery stays as is and runs radio, sink, lights, etc...   Does not run
fridge on 12VDC.
                                                                  Jay