I put a second Group 41 battery under my rear seat of my '85 camper.  I ran
both + and - leads right to the starter using 1AWG for the positive and 2
AWG for the ground.  I drilled two holes through the body just to the left
of the rear heater box. The positive cable was 31" long and worked out
perfectly.  The ground cable was 36".  I put the battery in a plastic
battery box that I picked up at Pep Boys.  Everything including the box and
cables was less than $40.  (I already had the battery.)  I left the other
battery under the front seat, so I have two Group 41 batteries hooked
together at the starter stud connection--the factory battery to starter
wire is still in place.  Cranking speed is noticeably better.   Future
plans will include installing a disconnect on the rear seat battery, which
would prevent it from being run down while camping.   For now, I wanted to
see what the effect would be on cranking speed.

                                                   Jay '85 Westy (The
Hindenburg)