RE: did you put the charger under your rear seat as well? Did you
use an enclosed battery box or a tray and a strap? How did you route
the cable to the starter battery? Gauge # of the wire?

Charger is under the back seat with the battery and permanently plugged into
the 110v oulet down by the floor, will be hard wired to the 110v lines when
I get the chance to do the work.
Although and AGM (sealed) battery it is in a battery box, box is screwed to
the floor and the lid of the battery box held on with the strap provided.
That protects the terminals from coming in contact with anything else under
the back seat.
Two #2 cables go from the aux battery through the 'hidden compartment then
behind the fridge/sink/stove cabinet and up to the original aux battery
compartment.  The ground is grounded there and the hot goes to a large
terminal post.  The connection to the starting battery (attached to the
1315) is the original, about #10 or #12.  That will be replaced with a #4
but snaking that in between the battery boxes seems to be a task and has
been put off for now.  When it is in I'll add a switch that will allow the
two batteries to be connected for starting, a feature of the 1315, not
implemented yet.
I chose to route the aux up to the old battery compartment because I already
had wiring in place there to a fuse block for all add ons (inverter, CB, 12v
outlets, etc) and initially I just connected the aux battery to the same
cables that were connected to the original small aux battery.  That changed
after I was sure everything was working the way I wanted, changed to a
better and more permanent setup.